Members' Capital (Narrative) (Details) - shares	12 Months Ended
Members' Capital (Narrative) (Details) - shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Members Capital Account [Line Items]
Members capital account, Units issued	2,992,482	2,992,482
Members capital account, Units outstanding	2,992,482	2,992,482
Potential repurchase price of Units as percentage of holder's capital account	100.00%
Allocation of net income and net losses	100.00%
Other Members [Member]
Other Members Capital Account [Line Items]
Members capital account, Units issued	2,992,482
Members capital account, Units outstanding	2,992,482	2,992,482	2,992,482
Members capital account, Units authorized	20,000,000	20,000,000
Allocation of net income and net losses from commencement until initial closing date	1.00%
Allocation of net income and net losses commencing with initial closing date	92.50%
Percentage of fund distributions	92.50%
Managing Member [Member]
Other Members Capital Account [Line Items]
Members capital account, Units issued	50
Allocation of net income and net losses from commencement until initial closing date	99.00%
Allocation of net income and net losses commencing with initial closing date	7.50%
Percentage of fund distributions	7.50%

ATEL 12, LLC 10-K