Loans Receivable, Net - 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, 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
Total Loan Exposure	$ 15,053,372	$ 14,740,844
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	122	125
Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 14,508,735	$ 14,191,200
Risk Rating 1 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Exposure	$ 255,899	$ 182,740
Risk Rating 1 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	2
Risk Rating 1 [Member] | Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 255,229	$ 181,366
Risk Rating 2 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Exposure	$ 4,350,775	$ 3,950,025
Risk Rating 2 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	37	38
Risk Rating 2 [Member] | Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 4,275,769	$ 3,860,432
Risk Rating 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Exposure	$ 10,446,698	$ 10,608,079
Risk Rating 3 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	82	85
Risk Rating 3 [Member] | Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,977,737	$ 10,149,402