Loans Receivable, Net - Overall Statistics for Loans Receivable Portfolio (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	122	125
Net book value	$ 14,508,735	$ 14,191,200
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	122	125
Principal balance	$ 14,603,831	$ 14,293,970
Unfunded loan commitments	$ 3,123,059	$ 3,405,945
Weighted-average maximum maturity (years)	3 years 9 months 18 days	3 years 10 months 24 days
Loans Receivable [Member] | LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average cash coupon, rate	5.63%	5.67%
Weighted-average all-in yield, rate	5.99%	6.00%
Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 14,508,735	$ 14,191,200