Secured Debt Agreements, Net - Additional Information (Detail) € in Millions	3 Months Ended		6 Months Ended				12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Facility	Jun. 30, 2018 GBP (£) Facility	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2018 EUR (€)	Jun. 30, 2018 GBP (£)	Dec. 31, 2017 GBP (£)
Debt Instrument [Line Items]
Aggregate borrowings	$ 4,649,246,000		$ 4,649,246,000
Basis spread on debt obligation, in percent			1.79%	1.79%
Weighted-average advance rate			79.10%	79.10%
Secured debt agreements borrowings outstanding					$ 38,456,000		$ 38,456,000
Maximum Facility Size	660,400,000		$ 660,400,000		675,700,000		675,700,000		£ 500,000,000	£ 500,000,000
Interest expense	87,733,000	$ 54,352,000	157,722,000			$ 101,026,000
Secured debt agreements borrowings outstanding	7,044,550,000		7,044,550,000		5,290,536,000		5,290,536,000
Covenants, minimum tangible net worth	$ 2,200,000,000		$ 2,200,000,000
Covenants, percentage of tangible assets on cash proceeds from equity issuances			75.00%	75.00%
Covenants, percentage of recourse indebtedness	5.00%		5.00%					5.00%	5.00%
Maximum [Member]
Debt Instrument [Line Items]
Covenants, indebtedness to total assets, in percent	83.33%		83.33%					83.33%	83.33%
Minimum [Member]
Debt Instrument [Line Items]
Covenants, EBITDA to fixed charges, in percent			140.00%	140.00%
Covenants, minimum cash liquidity amount	$ 10,000,000		$ 10,000,000
Asset-Specific Financings [Member]
Debt Instrument [Line Items]
Weighted-average outstanding balance			1,100,000,000		525,500,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Weighted-average outstanding balance			57,000,000		59,900,000
Interest expense			885,000		2,200,000
Amortization of deferred fees and expenses			511,000		497,000
Secured debt agreements borrowings outstanding	0		0		0		0
Revolving Credit Facility [Member] | Barclays [Member]
Debt Instrument [Line Items]
Maximum Facility Size	250,000,000		$ 250,000,000
First mortgage bridge to term financing maximum period			6 months	6 months
Credit Facilities [Member]
Debt Instrument [Line Items]
Additional credit capacity | £				£ 250,000,000
Weighted-average outstanding balance			$ 4,300,000,000		4,400,000,000
Aggregate borrowings	4,649,246,000		$ 4,649,246,000		4,068,249,000		$ 4,068,249,000
Weighted-average advance rate			79.10%	79.10%			78.70%
Weighted-average initial maturity			1 year 7 months 6 days	1 year 7 months 6 days			1 year 6 months
Maximum Facility Size	8,327,710,000		$ 8,327,710,000		7,155,975,000		$ 7,155,975,000
Secured debt agreements borrowings outstanding	4,649,246,000		4,649,246,000		$ 4,068,249,000		$ 4,068,249,000
Credit Facilities [Member] | Barclays [Member]
Debt Instrument [Line Items]
Aggregate borrowings	496,480,000		496,480,000
Maximum Facility Size	1,000,000,000		$ 1,000,000,000
New Credit Facilities [Member]
Debt Instrument [Line Items]
Number of new credit facility | Facility			1	1
Additional credit capacity | £				£ 1,000,000,000
Asset-Specific Financings [Member]
Debt Instrument [Line Items]
Secured debt agreements borrowings outstanding	414,000,000		$ 414,000,000					€ 800.0
Asset-Specific Financings [Member] | Senior Loan [Member]
Debt Instrument [Line Items]
Secured loan	$ 517,500,000		$ 517,500,000					€ 1,000.0
LIBOR [Member] | Credit Facilities [Member] | Weighted-Average Cash Coupon [Member]
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent			1.79%	1.79%			1.90%
LIBOR [Member] | Credit Facilities [Member] | Weighted-Average All-in Cost of Credit [Member]
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent			1.98%	1.98%			2.12%