Fair Value Measurements (Assets Measured at Fair Value on a Non-recurring Basis) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	$ 4,473		$ 4,473		$ 3,145
Total assets at fair value	4,486		4,486		3,217
Net gain (loss) on loan sales	66	$ 90	114	$ 165
Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repossessed assets	9		9		14
Total assets at fair value	39		39		48
Fair value losses on repossessed assets			0		(2)
Net gain (loss) on loan sales			(5)		(32)
Nonrecurring | Residential first mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	9		9		9
Impaired loans held-for-investment	21		21		25
Nonrecurring | Residential first mortgage loans | Other non-interest income
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value losses on loans (less than for $1 million)			(1)		(2)
Nonrecurring | Residential first mortgage loans | Provision for loan losses
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value losses on loans (less than for $1 million)			(4)		(28)
Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repossessed assets	0		0		0
Total assets at fair value	9		9		9
Nonrecurring | Level 2 | Residential first mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	9		9		9
Impaired loans held-for-investment	0		0		0
Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repossessed assets	9		9		14
Total assets at fair value	30		30		39
Nonrecurring | Level 3 | Residential first mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	0		0		0
Impaired loans held-for-investment	$ 21		$ 21		$ 25