Loans - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 66,756,000,000	$ 67,593,000,000	$ 66,756,000,000	$ 67,593,000,000	$ 72,563,000,000
Net deferred origination costs (fees)	81,000,000		81,000,000		81,000,000
Net unamortized premium (discount)	10,000,000		10,000,000		8,000,000
Commercial loans removed from TDR classification	0	0	0	0
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	47,183,000,000		47,183,000,000		53,035,000,000
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	19,573,000,000		19,573,000,000		19,528,000,000
Affiliated Entity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 1,557,000,000	$ 1,820,000,000	$ 1,557,000,000	$ 1,820,000,000	$ 6,750,000,000