Fair Value Measurement and Fair Value of Financial Instruments - Financial Assets Measured at Fair Value on Nonrecurring Basis (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Loans:
Impaired loans	$ 777		$ 777		$ 865
Loss Impaired loans	13	$ 5	14	$ 21
Other assets:
Computer software, impairments	0	0	2	3
Gain (loss) on loans held for sale		0		2
Gain (loss) on private equity investments	0		2
Gain (loss) on equity method investment		0		(2)
Impairment of Consolidated Variable Interest Entity Low Income Housing Credit Investments	(8)	(8)	(8)	(8)
Loss, Total	(23)	(13)	(28)	(32)
Gain (Loss) on Energy Investments	(2)		(2)
Fair Value Measurements, Nonrecurring
Loans:
Impaired loans	122	124	122	124
Other assets:
Software	0	0	0	0
Variable Interest Entity, Consolidated, Carrying Amount, Assets	54	91	54	91
Loans held for sale		7		7
Private equity investments	10		10
Renewable energy investment		0		0
Total	188	222	188	222
Renewable Energy Investment	2		2
Fair Value Measurements, Nonrecurring | Level 1
Loans:
Impaired loans	0	0	0	0
Other assets:
Software	0	0	0	0
Variable Interest Entity, Consolidated, Carrying Amount, Assets	0	0	0	0
Loans held for sale		0		0
Private equity investments	0		0
Renewable energy investment		0		0
Total	0	0	0	0
Renewable Energy Investment	0		0
Fair Value Measurements, Nonrecurring | Level 2
Loans:
Impaired loans	0	0	0	0
Other assets:
Software	0	0	0	0
Variable Interest Entity, Consolidated, Carrying Amount, Assets	0	0	0	0
Loans held for sale		0		0
Private equity investments	0		0
Renewable energy investment		0		0
Total	0	0	0	0
Renewable Energy Investment	0		0
Fair Value Measurements, Nonrecurring | Level 3
Loans:
Impaired loans	122	124	122	124
Other assets:
Software	0	0	0	0
Variable Interest Entity, Consolidated, Carrying Amount, Assets	54	91	54	91
Loans held for sale		7		7
Private equity investments	10		10
Renewable energy investment		0		0
Total	188	$ 222	188	$ 222
Renewable Energy Investment	$ 2		$ 2