Loans Receivable, Net - Overall Statistics for Loans Receivable Portfolio (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	128	125
Net book value	$ 14,755,072	$ 14,191,200
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	128	125
Principal balance	$ 14,849,556	$ 14,293,970
Unfunded loan commitments	$ 4,724,809	$ 3,405,945
Weighted-average maximum maturity (years)	3 years 7 months 6 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	5.07%	5.67%
Weighted-average all-in yield, rate	5.42%	6.00%
Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 14,755,072	$ 14,191,200