Fair Value Measurements - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) Source	Mar. 31, 2015 USD ($)	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
Securities issued by financial institutions and other entities backed by trust preferred securities at cost	$ 28		$ 28
Fair value of securities issued by financial institutions and other entities backed by trust preferred securities	45		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	27	$ 8
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	226	101	210
Assets taken in foreclosure of defaulted loans measured at fair value on a nonrecurring basis	62	11
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	127	67	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	$ 99	$ 34	$ 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	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%