Secured Debt Agreements, Net - Additional Information (Detail) € in Millions, £ in Millions		3 Months Ended				9 Months Ended				12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2015 USD ($) ExtensionOptions	Sep. 30, 2017 USD ($) Facility	Sep. 30, 2017 GBP (£) Facility	Sep. 30, 2017 EUR (€) Facility	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	May 31, 2017 USD ($)
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent						1.88%	1.88%	1.88%
Weighted-average advance rate						78.80%	78.80%	78.80%
Secured debt agreements borrowings outstanding	$ 6,079,135,000	$ 6,079,135,000				$ 6,079,135,000				$ 5,716,354,000
Interest expense		67,891,000		$ 45,373,000		168,917,000			$ 139,819,000
Secured debt agreements borrowings outstanding	6,096,597,000	6,096,597,000				6,096,597,000				5,731,626,000
Covenants, minimum tangible net worth	$ 1,900,000,000	$ 1,900,000,000				$ 1,900,000,000
Covenants, percentage of tangible assets on cash proceeds from equity issuances						75.00%	75.00%	75.00%
Covenants, percentage of recourse indebtedness	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%
Minimum [Member]
Debt Instrument [Line Items]
Covenants, EBITDA to fixed charges, in percent						140.00%	140.00%	140.00%
Covenants, minimum cash liquidity amount	$ 10,000,000	$ 10,000,000				$ 10,000,000
Asset-Specific Financings [Member]
Debt Instrument [Line Items]
Weighted-average outstanding balance						596,200,000			557,900,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Weighted-average outstanding balance						23,500,000			19,500,000
Interest expense						1,700,000			915,000
Amortization of deferred fees and expenses						575,000			248,000
Secured debt agreements borrowings outstanding	101,750,000	101,750,000		$ 0		101,750,000			0
Revolving Credit Facility [Member] | Barclays [Member]
Debt Instrument [Line Items]
Maximum Facility Size			$ 250,000,000								$ 125,000,000
First mortgage bridge to term financing maximum period			6 months
GE Portfolio Acquisition Facility [Member]
Debt Instrument [Line Items]
Secured debt agreements borrowings outstanding	1,090,946,000	1,090,946,000				1,090,946,000				1,479,582,000
GE Portfolio Acquisition Facility [Member] | Wells Fargo [Member]
Debt Instrument [Line Items]
Maximum Facility Size	1,200,000,000	1,200,000,000				1,200,000,000
Approximate credit line, outstanding amount	1,100,000,000
Amount available for potential future fundings of loan	129,400,000	129,400,000				$ 129,400,000
GE Portfolio Acquisition Facility [Member] | Asset Specific Borrowings [Member]
Debt Instrument [Line Items]
Weighted average rate					80.00%
Maturity date						May 20, 2020	May 20, 2020	May 20, 2020
Borrowings maturity term						The asset-specific borrowings are term matched to the underlying collateral assets with an outside maturity date of May 20, 2020, which may be extended pursuant to two one-year extension options.	The asset-specific borrowings are term matched to the underlying collateral assets with an outside maturity date of May 20, 2020, which may be extended pursuant to two one-year extension options.	The asset-specific borrowings are term matched to the underlying collateral assets with an outside maturity date of May 20, 2020, which may be extended pursuant to two one-year extension options.
Number of extension options | ExtensionOptions					2
Term of maturity date					1 year
GE Portfolio Acquisition Facility [Member] | Asset Specific Borrowings [Member] | Maximum [Member]
Debt Instrument [Line Items]
Percentage of guarantee rate					25.00%
Guarantee of outstanding borrowings					$ 250,000,000
GE Portfolio Acquisition Facility [Member] | Weighted-Average All-in Cost of Credit [Member] | Asset Specific Borrowings [Member]
Debt Instrument [Line Items]
Secured debt agreements borrowings outstanding	1,100,000,000	1,100,000,000				$ 1,100,000,000				1,500,000,000
Credit Facilities [Member]
Debt Instrument [Line Items]
Additional credit capacity							£ 250.0	€ 250.0
Number of revolving repurchase facilities closed | Facility						2	2	2
Weighted-average outstanding balance						$ 4,000,000,000			$ 2,900,000,000
Aggregate borrowings	4,400,000,000	4,400,000,000				$ 4,400,000,000				$ 3,600,000,000
Weighted-average advance rate						78.80%	78.80%	78.80%		79.10%
Weighted-average initial maturity						1 year 6 months	1 year 6 months	1 year 6 months		1 year 6 months
Maximum Facility Size	7,137,810,000	7,137,810,000				$ 7,137,810,000				$ 5,479,180,000
Amount available for potential future fundings of loan	602,705,000	602,705,000				602,705,000				541,743,000
Secured debt agreements borrowings outstanding	4,386,645,000	4,386,645,000				$ 4,386,645,000				3,572,837,000
Credit Facilities [Member] | Joint Venture [Member] | Multifamily [Member]
Debt Instrument [Line Items]
Number of new credit facility | Facility						2	2	2
Additional credit capacity						$ 450,000,000
Credit Facilities [Member] | Wells Fargo [Member]
Debt Instrument [Line Items]
Maximum Facility Size	2,000,000,000	2,000,000,000				2,000,000,000				2,000,000,000
Amount available for potential future fundings of loan	$ 326,003,000	$ 326,003,000				$ 326,003,000				$ 232,209,000
Borrowings maturity term						Term matched	Term matched	Term matched
Credit Facilities [Member] | Asset-Specific Financings [Member]
Debt Instrument [Line Items]
Additional credit capacity						$ 500,000,000
Base Rate [Member] | GE Portfolio Acquisition Facility [Member] | Asset Specific Borrowings [Member] | Primary [Member]
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent					1.75%
Base Rate [Member] | GE Portfolio Acquisition Facility [Member] | Asset Specific Borrowings [Member] | Primary [Member] | Year Four [Member]
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent					1.80%
Base Rate [Member] | GE Portfolio Acquisition Facility [Member] | Asset Specific Borrowings [Member] | Primary [Member] | Year Five [Member]
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent					1.85%
LIBOR [Member] | GE Portfolio Acquisition Facility [Member] | Weighted-Average All-in Cost of Credit [Member] | Asset Specific Borrowings [Member]
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent						1.75%	1.75%	1.75%		1.83%
LIBOR [Member] | Credit Facilities [Member] | Weighted-Average Cash Coupon [Member]
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent						1.88%	1.88%	1.88%		1.82%
LIBOR [Member] | Credit Facilities [Member] | Weighted-Average All-in Cost of Credit [Member]
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent						2.09%	2.09%	2.09%		2.02%