Fair Value Measurement and Fair Value of Financial Instruments - Assets Measured on Nonrecurring Basis (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Loans:
Impaired loans		$ 682	$ 730
Gains Losses On Impaired Loans		(20)	(106)
Other assets:
OREO		1	1
Goodwill		1,764	3,301
Goodwill, Impairment Loss	$ (1,600)	(1,614)	0	$ 0
Other assets		10,200	9,620
Gains Losses on Other Assets		(36)	(45)
Total Gains Losses on Assets		(1,670)	(151)
Fair Value, Nonrecurring [Member]
Loans:
Impaired loans		64	151
Other assets:
Total		2,129	290
Goodwill		1,764
Other assets		301	139
Fair Value, Nonrecurring [Member] | Level 3
Loans:
Impaired loans		64	151
Other assets:
Total		2,129	290
Goodwill		1,764
Other assets		301	139
Fair Value, Nonrecurring [Member] | Level 2
Loans:
Impaired loans		0	0
Other assets:
Total		0	0
Goodwill		0
Other assets		0	0
Fair Value, Nonrecurring [Member] | Level 1
Loans:
Impaired loans		0	0
Other assets:
Total		0	0
Goodwill		0
Other assets		$ 0	$ 0