Fair Value Measurements - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) Source	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) Source	Sep. 30, 2014 USD ($)	Dec. 31, 2014 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		$ 28		$ 30
Fair value of securities issued by financial institutions and other entities backed by trust preferred securities	50		50		50
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	11	$ 23	53	$ 46
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	177	196	177	196	173
Assets taken in foreclosure of defaulted loans measured at fair value on a nonrecurring basis	15	21	15	21
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	106	112	106	112	94
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	$ 71	$ 84	$ 71	$ 84	$ 79
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			10.00%
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			90.00%
Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount margin over LIBOR			8.00%