Fair Value (Fair Value Measurements - Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Investment in Any One Industry Did Not Exceed Percentage	13.00%	14.00%
Equity securities	$ 514	$ 518
Available-for-sale securities	24,122	22,303
Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	24,122	22,303
Derivative assets	70	39
Assets, fair value	24,723	22,878
Derivative liabilities	(70)	(41)
Liabilities, fair value	70	41
Fair value, measurements, recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	(69)	(39)
Fair value, measurements, recurring | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	(1)	(2)
Fair value, measurements, recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	514	518
Fair value, measurements, recurring | US Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,904	1,777
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	865	854
Fair value, measurements, recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	155	154
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	16,014	14,291
Fair value, measurements, recurring | Mortgage-backed residential
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	2,561	2,494
Fair value, measurements, recurring | Mortgage-backed commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	631	541
Fair value, measurements, recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	733	936
Fair value, measurements, recurring | Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,259	1,256
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	13	13
Fair value, measurements, recurring | Retained interest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Retained interest, fair value	4	5
Fair value, measurements, recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	69	39
Fair value, measurements, recurring | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1
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	1,910	1,785
Derivative assets	0	0
Assets, fair value	2,413	2,303
Derivative liabilities	0	0
Liabilities, 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 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	0
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	503	518
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	1,903	1,777
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	8
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 | 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	22,212	20,518
Derivative assets	69	38
Assets, fair value	22,281	20,556
Derivative liabilities	(70)	(41)
Liabilities, fair value	70	41
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	(69)	(39)
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)	(2)
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	0
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	865	854
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	148	146
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	16,014	14,291
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	2,561	2,494
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	631	541
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	733	936
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	1,259	1,256
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	68	38
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	1
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	1	1
Assets, fair value	29	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	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	0
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 | 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	13	13
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	5
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	1	$ 1
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