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
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Loans:
Impaired loans	$ 928		$ 730
Loss Impaired loans	(15)	$ (1)
Other assets:
Computer software, impairments		(2)
Capitalized Computer Software, Impairments	(2)
Impairment of Premises and Equipment	(1)
Gain (loss) on private equity investments		(2)
Other Assets	9,378		$ 9,620
Other Asset Impairment Charges	1
Impairment of Renewable Energy Investments	(1)
Loss, Total	(20)	(5)
Fair Value Measurements, Nonrecurring
Loans:
Impaired loans	346	69
Other assets:
Software	0	0
Property, Plant and Equipment, Other, Net	0
Private equity investments		12
Other Assets	0
Total	353	81
Renewable Energy Investment	7
Fair Value Measurements, Nonrecurring | Level 1
Loans:
Impaired loans	0	0
Other assets:
Software	0	0
Property, Plant and Equipment, Other, Net	0
Private equity investments		0
Other Assets	0
Total	0	0
Renewable Energy Investment	0
Fair Value Measurements, Nonrecurring | Level 2
Loans:
Impaired loans	0	0
Other assets:
Software	0	0
Property, Plant and Equipment, Other, Net	0
Private equity investments		0
Other Assets	0
Total	0	0
Renewable Energy Investment	0
Fair Value Measurements, Nonrecurring | Level 3
Loans:
Impaired loans	346	69
Other assets:
Software	0	0
Property, Plant and Equipment, Other, Net	0
Private equity investments		12
Other Assets	0
Total	353	$ 81
Renewable Energy Investment	$ 7