Fair Value (Fair Value Measurements - Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	$ 536	$ 773
Available-for-sale securities	$ 27,630	$ 25,303
Investment in any one industry did not exceed percentage	15.00%	9.00%
Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 27,630	$ 25,303
Derivative assets	22	41
Assets, fair value	28,207	26,129
Derivative liabilities	(21)	(37)
Liabilities, fair value	21	37
Fair value, measurements, recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	(19)	(37)
Fair value, measurements, recurring | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	(1)
Fair value, measurements, recurring | Other contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	(1)
Fair value, measurements, recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	536	773
Fair value, measurements, recurring | US Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,942	1,851
Fair value, measurements, recurring | US States and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	781	802
Fair value, measurements, recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	172	145
Fair value, measurements, recurring | Agency mortgage-backed residential
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	18,844	17,138
Fair value, measurements, recurring | Agency mortgage-backed commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	320	3
Fair value, measurements, recurring | Mortgage-backed residential
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	2,886	2,686
Fair value, measurements, recurring | Mortgage-backed commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	723	714
Fair value, measurements, recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	668	723
Fair value, measurements, recurring | Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,294	1,241
Fair value, measurements, recurring | Loans held-for-sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held-for-sale, fair value	15	8
Fair value, measurements, recurring | Retained interest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Retained interest, fair value	4	4
Fair value, measurements, recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	22	37
Fair value, measurements, recurring | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		4
Fair value, measurements, recurring | Fair value, inputs, level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,948	1,857
Derivative assets	0	0
Assets, fair value	2,473	2,623
Derivative liabilities	(1)	0
Liabilities, fair value	1	0
Fair value, measurements, recurring | Fair value, inputs, level 1 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 1 | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0
Fair value, measurements, recurring | Fair value, inputs, level 1 | Other contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	(1)
Fair value, measurements, recurring | Fair value, inputs, level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	525	766
Fair value, measurements, recurring | Fair value, inputs, level 1 | US Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,941	1,850
Fair value, measurements, recurring | Fair value, inputs, level 1 | US States and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 1 | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	7	7
Fair value, measurements, recurring | Fair value, inputs, level 1 | Agency mortgage-backed residential
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 1 | Agency mortgage-backed commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 1 | Mortgage-backed residential
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 1 | Mortgage-backed commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 1 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 1 | Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 1 | Loans held-for-sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held-for-sale, fair value	0	0
Fair value, measurements, recurring | Fair value, inputs, level 1 | Retained interest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Retained interest, fair value	0	0
Fair value, measurements, recurring | Fair value, inputs, level 1 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Fair value, measurements, recurring | Fair value, inputs, level 1 | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0
Fair value, measurements, recurring | Fair value, inputs, level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	25,682	23,446
Derivative assets	20	41
Assets, fair value	25,702	23,487
Derivative liabilities	(20)	(37)
Liabilities, fair value	20	37
Fair value, measurements, recurring | Fair value, inputs, level 2 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	(19)	(37)
Fair value, measurements, recurring | Fair value, inputs, level 2 | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	(1)
Fair value, measurements, recurring | Fair value, inputs, level 2 | Other contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0
Fair value, measurements, recurring | Fair value, inputs, level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 2 | US Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1	1
Fair value, measurements, recurring | Fair value, inputs, level 2 | US States and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	781	802
Fair value, measurements, recurring | Fair value, inputs, level 2 | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	165	138
Fair value, measurements, recurring | Fair value, inputs, level 2 | Agency mortgage-backed residential
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	18,844	17,138
Fair value, measurements, recurring | Fair value, inputs, level 2 | Agency mortgage-backed commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	320	3
Fair value, measurements, recurring | Fair value, inputs, level 2 | Mortgage-backed residential
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	2,886	2,686
Fair value, measurements, recurring | Fair value, inputs, level 2 | Mortgage-backed commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	723	714
Fair value, measurements, recurring | Fair value, inputs, level 2 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	668	723
Fair value, measurements, recurring | Fair value, inputs, level 2 | Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,294	1,241
Fair value, measurements, recurring | Fair value, inputs, level 2 | Loans held-for-sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held-for-sale, fair value	0	0
Fair value, measurements, recurring | Fair value, inputs, level 2 | Retained interest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Retained interest, fair value	0	0
Fair value, measurements, recurring | Fair value, inputs, level 2 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	20	37
Fair value, measurements, recurring | Fair value, inputs, level 2 | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		4
Fair value, measurements, recurring | Fair value, inputs, level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Derivative assets	2	0
Assets, fair value	32	19
Derivative liabilities	0	0
Liabilities, fair value	0	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | Other contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	11	7
Fair value, measurements, recurring | Fair value, inputs, level 3 | US Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | US States and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | Agency mortgage-backed residential
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | Agency mortgage-backed commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | Mortgage-backed residential
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | Mortgage-backed commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | Loans held-for-sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held-for-sale, fair value	15	8
Fair value, measurements, recurring | Fair value, inputs, level 3 | Retained interest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Retained interest, fair value	4	4
Fair value, measurements, recurring | Fair value, inputs, level 3 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	$ 2	0
Fair value, measurements, recurring | Fair value, inputs, level 3 | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		$ 0