Loans Receivable, Net - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan	Dec. 31, 2016 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	125	110
Net book value	$ 14,191,200	$ 10,056,732
Total Loan Exposure	$ 14,740,844	$ 11,093,608
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	125	110
Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 14,191,200	$ 10,056,732	$ 8,692,978
Risk Rating 1 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Exposure	$ 182,740	$ 31,890
Risk Rating 1 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	1
Risk Rating 1 [Member] | Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 181,366	$ 31,842
Risk Rating 2 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Exposure	$ 3,950,025	$ 3,521,701
Risk Rating 2 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	38	41
Risk Rating 2 [Member] | Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 3,860,432	$ 3,512,709
Risk Rating 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Exposure	$ 10,608,079	$ 7,519,465
Risk Rating 3 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	85	67
Risk Rating 3 [Member] | Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 10,149,402	$ 6,491,617
Risk Rating 4 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Exposure		$ 20,552
Risk Rating 4 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan		1
Risk Rating 4 [Member] | Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value		$ 20,564