Significant Accounting Policies - Additional Information (Detail) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Schedule Of Significant Accounting Policies [Line Items]
Real estate acquired in settlement of residential mortgage loans	$ 0	$ 0
Outstanding impaired loans, nonaccrual loans, and loans in default	0	0
Allowance for loan losses	$ 0	0
Minimum
Schedule Of Significant Accounting Policies [Line Items]
Senior secured loans, maturity period	3 years
Maximum
Schedule Of Significant Accounting Policies [Line Items]
Senior secured loans, maturity period	8 years
Interest Rate Swaps
Schedule Of Significant Accounting Policies [Line Items]
Cash on margin with counterparty to interest rate swaps	$ 590,100,000	$ 1,400,000,000
Centrally cleared interest rate swaps at fair value, reduction	$ 799,800,000