Loans Held for Sale - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for sale			$ 715	$ 1,809
Transfer of loans to held for sale, net			1,253	1,588	$ 1,850
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for sale			648	844
Valuation allowance			10	55
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for sale			67	965
Valuation allowance			5	57
Commercial Loans - Fair Value Option [Member] | Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for sale			471	725
Commercial Loans - Transferred to Held for Sale [Member] | Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for sale			62	102
Commercial Real Estate [Member] | Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for sale			115	17
Loans held for sale, sold, unpaid principal balance				1,161
Gain (loss) on sale of loans held for sale				(3)
Loans transferred to held for investment				612
Residential Mortgage [Member] | Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for sale			6	890
Lower of amortized cost or fair value adjustment attributed to non-credit factors				45
Transfer of loans to held for sale, unpaid principal balance				568
Transfer of loans to held for sale, net				473
Loans held for sale, sold, unpaid principal balance	$ 364
Loans held for sale, sold, at aggregate carrying value	276
Gain (loss) on sale of loans held for sale	$ (2)
Residential Mortgage And Home Equity [Member] | Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transfer of loans to held for sale, unpaid principal balance				648
Transfer of loans to held for sale, net				628
Lower of cost or fair value adjustment attributed to credit factors			(1)
Loans held for sale, sold, unpaid principal balance			581
Loans held for sale, sold, at aggregate carrying value			536
Gain (loss) on sale of loans held for sale			50
Subprime [Member] | Residential Mortgage [Member] | Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for sale			2	3
Other Income [Member] | Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Lower of amortized cost or fair value adjustment attributed to non-credit factors			$ (5)	35	$ 16
Other Income [Member] | Residential Mortgage [Member] | Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Additional lower of amortized cost or fair value adjustment		$ (5)		8
Other Income [Member] | Residential Mortgage And Home Equity [Member] | Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Additional lower of amortized cost or fair value adjustment				4
Provision for Credit Losses [Member] | Residential Mortgage And Home Equity [Member] | Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Lower of cost or fair value adjustment attributed to credit factors				$ 11