Secured Debt Agreements, Net - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) Facility	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Secured debt agreements borrowings outstanding	$ 491,598,000		$ 491,598,000
Interest expense	116,891,000	$ 87,733,000	235,579,000	$ 157,722,000
Covenants, minimum tangible net worth	$ 2,800,000,000		$ 2,800,000,000
Covenants, percentage of tangible assets on cash proceeds from equity issuances			75.00%
Covenants, percentage of recourse indebtedness	5.00%		5.00%
Maximum [Member]
Debt Instrument [Line Items]
Covenants, indebtedness to total assets, in percent	83.33%		83.33%
Minimum [Member]
Debt Instrument [Line Items]
Covenants, EBITDA to fixed charges, in percent			1.40%
Covenants, minimum cash liquidity amount	$ 10,000,000.0		$ 10,000,000.0
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Weighted-average outstanding balance			28,800,000		$ 31,700,000
Secured debt agreements borrowings outstanding	0		0		43,800,000
Interest expense			1,600,000		1,700,000
Amortization of deferred fees and expenses			522,000		551,000
Revolving Credit Facility [Member] | Barclays [Member]
Debt Instrument [Line Items]
Maximum Facility Size	250,000,000.0		250,000,000.0
Credit Facilities [Member]
Debt Instrument [Line Items]
Weighted-average outstanding balance			8,800,000,000		7,100,000,000
Aggregate borrowings	8,065,675,000		$ 8,065,675,000		$ 8,870,897,000
Basis spread on debt obligation, in percent			1.71%
Weighted-average advance rate			79.50%		79.50%
Weighted-average initial maturity			3 years 2 months 12 days		3 years 6 months
Credit Facilities [Member] | Joint Venture [Member]
Debt Instrument [Line Items]
Number of new credit facility | Facility			2
Additional credit capacity			$ 310,000,000.0
Credit Facilities [Member] | Barclays [Member]
Debt Instrument [Line Items]
Aggregate borrowings	$ 902,146,000		902,146,000		$ 890,620,000
Asset-Specific Financings [Member]
Debt Instrument [Line Items]
Weighted-average outstanding balance			$ 106,300,000		$ 50,400,000
LIBOR [Member] | Credit Facilities [Member] | Weighted-Average Cash Coupon [Member]
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent					1.72%
LIBOR [Member] | Credit Facilities [Member] | Weighted-Average All-in Cost of Credit [Member]
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent			1.89%		1.90%