Comprehensive Income - Schedule of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 7,602.8	$ 7,947.2	$ 6,533.9
Other comprehensive income (loss) before reclassifications	(59.8)	69.3	79.1
Amounts reclassified from AOCL	12.2	8.4	10.8
Total other comprehensive (loss) income, net of tax	(47.6)	77.7	89.9
Ending Balance	7,901.8	7,602.8	7,947.2
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(89.2)	(166.9)	(256.8)
Total other comprehensive (loss) income, net of tax	(47.6)	77.7	89.9
Ending Balance	(136.8)	(89.2)	(166.9)
Pension and Other Postretirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(181.8)	(176.2)	(192.5)
Other comprehensive income (loss) before reclassifications	61.7	(12.0)	9.7
Amounts reclassified from AOCL	7.0	6.4	6.6
Total other comprehensive (loss) income, net of tax	68.7	(5.6)	16.3
Ending Balance	(113.1)	(181.8)	(176.2)
Net Unrealized Gains (Losses) on Debt Securities Available-for-Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	103.1	20.8	(47.0)
Other comprehensive income (loss) before reclassifications	(122.4)	82.3	67.9
Amounts reclassified from AOCL	0.0	0.0	(0.1)
Total other comprehensive (loss) income, net of tax	(122.4)	82.3	67.8
Ending Balance	(19.3)	103.1	20.8
Net Unrealized Gains (Losses) on Debt Securities Transferred to Held-to-Maturity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(8.2)	(11.8)	(15.3)
Other comprehensive income (loss) before reclassifications	0.0	0.0	0.0
Amounts reclassified from AOCL	3.8	3.6	3.5
Total other comprehensive (loss) income, net of tax	3.8	3.6	3.5
Ending Balance	(4.4)	(8.2)	(11.8)
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(2.3)	0.3	(2.0)
Other comprehensive income (loss) before reclassifications	0.9	(1.0)	1.5
Amounts reclassified from AOCL	1.4	(1.6)	0.8
Total other comprehensive (loss) income, net of tax	2.3	(2.6)	2.3
Ending Balance	$ 0.0	$ (2.3)	$ 0.3

People's United Financial, Inc. 10-K