Debt Securities Available for Sale and Debt Securities Held to Maturity - Fair value and gross unrealized losses of available for sale debt securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	$ 477,633	$ 128,249
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	2,577	473
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	457,093	532,650
Available-for-sale securities, securities in a loss position for 12 months or longer, unrealized losses	14,431	15,091
Available-for-sale securities, fair value	934,726	660,899
Available-for-sale securities, unrealized losses	17,008	15,564
U.S. Treasury and other U.S. government agencies
Debt securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	28,410	6,041
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	790	60
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	319,805	335,296
Available-for-sale securities, securities in a loss position for 12 months or longer, unrealized losses	13,483	14,119
Available-for-sale securities, fair value	348,215	341,337
Available-for-sale securities, unrealized losses	14,273	14,179
Agency mortgage-backed securities
Debt securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	32,699	25,710
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	1,286	299
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	34,710	35,326
Available-for-sale securities, securities in a loss position for 12 months or longer, unrealized losses	818	796
Available-for-sale securities, fair value	67,409	61,036
Available-for-sale securities, unrealized losses	2,104	1,095
Agency collateralized mortgage obligations
Debt securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	416,524	96,498
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	501	114
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	102,578	162,028
Available-for-sale securities, securities in a loss position for 12 months or longer, unrealized losses	130	176
Available-for-sale securities, fair value	519,102	258,526
Available-for-sale securities, unrealized losses	$ 631	$ 290

BBVA USA Bancshares, Inc. 10-Q