Loans - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 66,399,000,000	$ 68,063,000,000	$ 66,399,000,000	$ 68,063,000,000	$ 72,563,000,000
Net deferred origination costs (fees)	81,000,000		81,000,000		81,000,000
Net unamortized premium (discount)	8,000,000		8,000,000		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	46,843,000,000		46,843,000,000		53,035,000,000
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	19,556,000,000		19,556,000,000		19,528,000,000
Affiliated Entity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 1,895,000,000	$ 1,423,000,000	$ 1,895,000,000	$ 1,423,000,000	$ 6,750,000,000