Fair Value Measurement - Schedule of Assets Measured at Fair Value on Nonrecurring Basis (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net loans held for investment	$ 0	$ 0
Loans held for sale	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net loans held for investment	0	0
Loans held for sale	1,308	860
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net loans held for investment	224,489	222,007
Loans held for sale	0	73
Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net loans held for investment	716	362
Loans held for sale	6	149
Other Assets, Fair Value Disclosure	39	92
Total	761	603
Nonrecurring | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed Properties and Repossessed Assets	39	54
Long-Lived Assets	0	38
Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net loans held for investment	0	0
Loans held for sale	0	0
Other Assets, Fair Value Disclosure	0	0
Total	0	0
Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net loans held for investment	0	0
Loans held for sale	6	149
Other Assets, Fair Value Disclosure	0	0
Total	6	149
Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net loans held for investment	716	362
Loans held for sale	0	0
Other Assets, Fair Value Disclosure	39	92
Total	$ 755	$ 454