Loans Receivable - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015 SecurityLoan	Dec. 31, 2014 SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans	60	60
Principal Balance	$ 4,977,468	$ 4,462,897
Net book value	4,943,383	4,428,500
Risk Rating One [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans	4	5
Principal Balance	405,957	209,961
Net book value	401,840	209,112
Risk Rating Two [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans	45	44
Principal Balance	3,741,108	3,339,972
Net book value	3,717,453	3,313,906
Risk Rating 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans	11	11
Principal Balance	830,403	912,964
Net book value	$ 824,090	$ 905,482