Fair Value of Financial Instruments - Additional Information (Detail) - Jun. 30, 2015 - USD ($) $ in Millions	Total	Total
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Dealer marks of securities	81.00%	81.00%
Percentage of carrying value for which dealer quotes have been received	92.00%	92.00%
Residential Subordinate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	$ 36	$ 59
Residential Senior Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	33	33
Residential Loans
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	2,780	5,180
Commercial Loans
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	258	350
Mortgage servicing rights
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	$ 32	$ 51
Mortgage servicing rights | Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Percentage difference of internal valuation than dealer marks	1.00%	1.00%