Secured Debt Agreements, Net - Additional Information (Detail)	3 Months Ended				12 Months Ended
	Mar. 31, 2018 USD ($) Facility	Mar. 31, 2018 GBP (£) Facility	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2018 GBP (£)	Dec. 31, 2017 GBP (£)
Debt Instrument [Line Items]
Aggregate borrowings	$ 3,851,609,000
Basis spread on debt obligation, in percent	1.84%	1.84%
Weighted-average advance rate	78.80%	78.80%
Secured debt agreements borrowings outstanding			$ 38,456,000		$ 38,456,000
Maximum Facility Size	$ 700,900,000		675,700,000		675,700,000	£ 500,000,000	£ 500,000,000
Interest expense	69,989,000			$ 46,674,000
Secured debt agreements borrowings outstanding	6,018,670,000		5,290,536,000		5,290,536,000
Covenants, minimum tangible net worth	$ 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%
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	140.00%	140.00%
Covenants, minimum cash liquidity amount	$ 10,000,000
Asset-Specific Financings [Member]
Debt Instrument [Line Items]
Weighted-average outstanding balance	632,600,000		517,700,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Weighted-average outstanding balance			52,000,000
Interest expense	532,000		1,100,000
Amortization of deferred fees and expenses	260,000		253,000
Secured debt agreements borrowings outstanding	0		0		0
Revolving Credit Facility [Member] | Barclays [Member]
Debt Instrument [Line Items]
Maximum Facility Size	$ 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	$ 4,100,000,000		4,400,000,000
Aggregate borrowings	$ 3,851,609,000		4,068,249,000		$ 4,068,249,000
Weighted-average advance rate	78.80%	78.80%			78.70%
Weighted-average initial maturity	1 year 4 months 24 days	1 year 4 months 24 days			1 year 6 months
Maximum Facility Size	$ 8,392,900,000		7,155,975,000		$ 7,155,975,000
Secured debt agreements borrowings outstanding	3,851,609,000		$ 4,068,249,000		$ 4,068,249,000
Credit Facilities [Member] | Barclays [Member]
Debt Instrument [Line Items]
Maximum Facility Size	$ 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	$ 800,000,000
Asset-Specific Financings [Member] | Senior Loan [Member]
Debt Instrument [Line Items]
Secured loan	$ 1,000,000,000
LIBOR [Member] | Credit Facilities [Member] | Weighted-Average Cash Coupon [Member]
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent	1.84%	1.84%			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	2.07%	2.07%			2.12%