Loans Receivable - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) SecurityLoan	Dec. 31, 2015 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,313,763	$ 9,077,007
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	126	125
Principal Balance	$ 9,346,247	$ 9,108,361
Net book value	$ 9,313,763	$ 9,077,007
Risk Rating 1 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	13	12
Principal Balance	$ 915,990	$ 925,443
Net book value	$ 911,350	$ 919,991
Risk Rating 2 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	72	77
Principal Balance	$ 5,743,148	$ 5,948,922
Net book value	$ 5,729,541	$ 5,929,447
Risk Rating 3 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	40	35
Principal Balance	$ 2,633,081	$ 2,120,713
Net book value	2,619,223	2,114,531
Risk Rating 4 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal Balance		$ 113,300
Net book value	$ 53,600
Risk Rating 4 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	1	1
Principal Balance	$ 54,028	$ 113,283
Net book value	$ 53,649	$ 113,038