Fair Value Measurements - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) Source	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) Source	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Number of independent pricing sources used to value privately issued mortgage-backed securities valued with internal modeling | Source	2		2
Securities issued by financial institutions and other entities backed by trust preferred securities at cost					$ 28
Fair value of securities issued by financial institutions and other entities backed by trust preferred securities					47
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Change in fair value of nonrecurring fair value measured loans for charge-offs and impairment reserves	$ 20	$ 11	$ 42	$ 53
Fair Value, Measurements, Nonrecurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Loans measured at fair value on nonrecurring basis	249	177	249	177	210
Assets taken in foreclosure of defaulted loans measured at fair value on a nonrecurring basis	53	15	53	15
Fair Value, Measurements, Nonrecurring [Member] | Significant Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Loans measured at fair value on nonrecurring basis	144	106	144	106	106
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Loans measured at fair value on nonrecurring basis	$ 105	$ 71	$ 105	$ 71	$ 104
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Discount margin over LIBOR			4.00%
Discount rates for fair value estimations			15.00%
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Discount margin over LIBOR			10.00%
Discount rates for fair value estimations			92.00%
Weighted Average [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Discount margin over LIBOR			8.00%