Secured Financing Agreements (Details) $ in Thousands	1 Months Ended						9 Months Ended
	Sep. 30, 2017 USD ($) item	Aug. 31, 2017 USD ($) item	Jul. 31, 2017	Jun. 30, 2017	Mar. 31, 2017 USD ($) item	Feb. 28, 2017 USD ($) item	Sep. 30, 2017 USD ($) item	May 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Secured Financing Agreements
Principal Amount	$ 2,073,229						$ 2,073,229		$ 2,053,229
Unamortized deferred financing costs	(6,018)						(6,018)		(5,822)
Carrying Value	5,514,695						5,514,695		4,154,126
Lender 4 Repo 2 Facility
Secured Financing Agreements
Maximum borrowing capacity	600,000						600,000
Maximum facility size subject to certain conditions	1,000,000						1,000,000
Lender 6 Repo 1 Facility
Secured Financing Agreements
Maximum Facility Size	600,000	$ 500,000					$ 600,000
Lender 9 Repo 1 Facility | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							1.65%
Lender 10 Repo 1 Facility
Secured Financing Agreements
Maximum Facility Size					$ 125,000			$ 140,000
Maturity period					3 years
Number of extension options | item					2
Extended term / option					1 year
Lender 10 Repo 1 Facility | LIBOR
Secured Financing Agreements
Interest rate (as a percent)					2.00%
Lender 10 Repo 1 Facility | Maximum | LIBOR
Secured Financing Agreements
Interest rate (as a percent)					2.75%
Lender 11 Repo 1 Facility
Secured Financing Agreements
Maturity period				2 years
Extended term / option				1 year
Lender 11 Repo 1 Facility | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.75%
Lender 11 Repo 2 Facility
Secured Financing Agreements
Maximum Facility Size		$ 250,000
Maturity period		1 year
Number of extension options | item		4
Extended term / option		1 year
Lender 11 Repo 2 Facility | Minimum | LIBOR
Secured Financing Agreements
Interest rate (as a percent)		2.25%
Lender 11 Repo 2 Facility | Maximum | LIBOR
Secured Financing Agreements
Interest rate (as a percent)		2.75%
Lender 7 Secured Financing
Secured Financing Agreements
Maximum borrowing capacity	450,000						$ 450,000
Maximum facility size subject to certain conditions	$ 650,000						$ 650,000
MBS Repo 1 Facility
Secured Financing Agreements
Rolling maturity period							11 months
MBS Repo 3 Facility
Secured Financing Agreements
Rolling maturity period							12 months
Master Lease Portfolio
Secured Financing Agreements
Maturity period	10 years
Number of mortgage facilities executed | item	2
Master Lease Portfolio | Minimum | LIBOR
Secured Financing Agreements
Interest rate (as a percent)	4.36%						4.36%
Master Lease Portfolio | Maximum | LIBOR
Secured Financing Agreements
Interest rate (as a percent)	4.38%						4.38%
Investing and Servicing Segment Property Mortgages
Secured Financing Agreements
Maximum borrowing capacity	$ 38,300						$ 38,300
Number of mortgage facilities executed | item							2
Investing and Servicing Segment Property Mortgages | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.00%
Investing and Servicing Segment Property Mortgages | Weighted-average
Secured Financing Agreements
Maturity period							4 years 7 months 6 days
Medical Office Portfolio Mortgages
Secured Financing Agreements
Floor interest rate (as a percent)							0.25%
Medical Office Portfolio Mortgages | Additional Mortgage Facilities Acquired
Secured Financing Agreements
Maximum Facility Size						$ 7,300
Maturity period						5 years
Number of extension options | item						2
Extended term / option						12 months
Medical Office Portfolio Mortgages | Additional Mortgage Facilities Acquired | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)						2.50%
FHLB | Minimum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)			0.15%
FHLB | Maximum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)			0.34%
Secured financing agreements
Secured Financing Agreements
Pledged Asset Carrying Value	9,035,494						$ 9,035,494
Maximum Facility Size	9,344,411						9,344,411
Principal Amount	5,555,720						5,555,720		4,197,218
Unamortized premium (discount), net	2,579						2,579		2,640
Unamortized deferred financing costs	(43,604)						(43,604)		(45,732)
Secured financing agreements | Lender 1 Repo 1 Facility
Secured Financing Agreements
Pledged Asset Carrying Value	1,534,069						1,534,069
Maximum Facility Size	2,000,000						2,000,000
Principal Amount	1,170,141						$ 1,170,141		944,712
Secured financing agreements | Lender 1 Repo 1 Facility | Minimum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							1.75%
Secured financing agreements | Lender 1 Repo 1 Facility | Maximum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							5.75%
Secured financing agreements | Lender 2 Repo 1 Facility
Secured Financing Agreements
Pledged Asset Carrying Value	350,508						$ 350,508
Maximum Facility Size	500,000						500,000
Principal Amount	236,055						$ 236,055		132,941
Secured financing agreements | Lender 2 Repo 1 Facility | Minimum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							1.75%
Secured financing agreements | Lender 2 Repo 1 Facility | Maximum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.75%
Secured financing agreements | Lender 3 Repo I Facility
Secured Financing Agreements
Pledged Asset Carrying Value	110,086						$ 110,086
Maximum Facility Size	76,820						76,820
Principal Amount	76,820						$ 76,820		78,288
Secured financing agreements | Lender 3 Repo I Facility | Minimum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.75%
Secured financing agreements | Lender 3 Repo I Facility | Maximum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							3.10%
Secured financing agreements | Lender 4 Repo 2 Facility
Secured Financing Agreements
Pledged Asset Carrying Value	571,746						$ 571,746
Maximum Facility Size	1,000,000						1,000,000
Principal Amount	221,544						$ 221,544		166,394
Secured financing agreements | Lender 4 Repo 2 Facility | Minimum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.00%
Secured financing agreements | Lender 4 Repo 2 Facility | Maximum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							3.25%
Secured financing agreements | Lender 6 Repo 1 Facility
Secured Financing Agreements
Pledged Asset Carrying Value	724,164						$ 724,164
Maximum Facility Size	600,000						600,000
Principal Amount	475,555						$ 475,555		182,586
Secured financing agreements | Lender 6 Repo 1 Facility | Minimum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.50%
Secured financing agreements | Lender 6 Repo 1 Facility | Maximum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.75%
Secured financing agreements | Lender 6 Repo 2 Facility
Secured Financing Agreements
Pledged Asset Carrying Value	173,516						$ 173,516
Maximum Facility Size	121,280						121,280
Principal Amount	121,280						$ 121,280		121,509
Secured financing agreements | Lender 6 Repo 2 Facility | GBP LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.75%
Secured financing agreements | Lender 9 Repo 1 Facility
Secured Financing Agreements
Pledged Asset Carrying Value	340,620						$ 340,620
Maximum Facility Size	254,447						254,447
Principal Amount	254,447						254,447		283,575
Secured financing agreements | Lender 10 Repo 1 Facility
Secured Financing Agreements
Pledged Asset Carrying Value	169,733						169,733
Maximum Facility Size	140,000						140,000
Principal Amount	136,800						$ 136,800
Secured financing agreements | Lender 10 Repo 1 Facility | Minimum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.00%
Secured financing agreements | Lender 10 Repo 1 Facility | Maximum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.75%
Secured financing agreements | Lender 11 Repo 1 Facility
Secured Financing Agreements
Maximum Facility Size	200,000						$ 200,000
Secured financing agreements | Lender 11 Repo 2 Facility
Secured Financing Agreements
Maximum Facility Size	250,000						$ 250,000
Secured financing agreements | Lender 11 Repo 2 Facility | Minimum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.25%
Secured financing agreements | Lender 11 Repo 2 Facility | Maximum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.75%
Secured financing agreements | Lender 7 Secured Financing
Secured Financing Agreements
Pledged Asset Carrying Value	46,800						$ 46,800
Maximum Facility Size	650,000						$ 650,000
Secured financing agreements | Lender 7 Secured Financing | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.75%
Secured financing agreements | Lender 8 Secured Financing
Secured Financing Agreements
Pledged Asset Carrying Value	30,147						$ 30,147
Maximum Facility Size	75,000						75,000
Principal Amount	18,226						$ 18,226		43,555
Secured financing agreements | Lender 8 Secured Financing | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							4.00%
Secured financing agreements | Conduit Repo 2 Facility
Secured Financing Agreements
Pledged Asset Carrying Value	53,751						$ 53,751
Maximum Facility Size	150,000						150,000
Principal Amount	40,842						$ 40,842		14,944
Secured financing agreements | Conduit Repo 2 Facility | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.25%
Secured financing agreements | Conduit Repo 3 Facility
Secured Financing Agreements
Pledged Asset Carrying Value	136,254						$ 136,254
Maximum Facility Size	150,000						150,000
Principal Amount	103,678						$ 103,678
Secured financing agreements | Conduit Repo 3 Facility | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.10%
Secured financing agreements | Conduit Repo 4 Facility
Secured Financing Agreements
Maximum Facility Size	100,000						$ 100,000
Secured financing agreements | Conduit Repo 4 Facility | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.25%
Secured financing agreements | MBS Repo 1 Facility
Secured Financing Agreements
Pledged Asset Carrying Value	10,000						$ 10,000
Maximum Facility Size	6,510						6,510
Principal Amount	6,510						$ 6,510		21,052
Secured financing agreements | MBS Repo 1 Facility | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							1.90%
Secured financing agreements | MBS Repo 2 Facility
Secured Financing Agreements
Pledged Asset Carrying Value	261,066						$ 261,066
Maximum Facility Size	191,184						191,184
Principal Amount	191,184						$ 191,184		239,434
Secured financing agreements | MBS Repo 2 Facility | Minimum | LIBOR/EURIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.00%
Secured financing agreements | MBS Repo 2 Facility | Maximum | LIBOR/EURIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.95%
Secured financing agreements | MBS Repo 3 Facility
Secured Financing Agreements
Pledged Asset Carrying Value	384,546						$ 384,546
Maximum Facility Size	254,668						254,668
Principal Amount	254,668						$ 254,668		285,209
Secured financing agreements | MBS Repo 3 Facility | Minimum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							1.32%
Secured financing agreements | MBS Repo 3 Facility | Maximum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							1.95%
Secured financing agreements | MBS Repo 4 Facility
Secured Financing Agreements
Pledged Asset Carrying Value	179,384						$ 179,384
Maximum Facility Size	225,000						225,000
Principal Amount	2,000						$ 2,000		5,633
Current maturity period, relative to when the buyer delivers notice to the seller							270 days
Secured financing agreements | MBS Repo 4 Facility | Minimum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							1.20%
Secured financing agreements | MBS Repo 4 Facility | Maximum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							1.90%
Secured financing agreements | Master Lease Portfolio
Secured Financing Agreements
Pledged Asset Carrying Value	471,762						$ 471,762
Maximum Facility Size	265,900						265,900
Principal Amount	265,900						265,900
Secured financing agreements | Investing and Servicing Segment Property Mortgages
Secured Financing Agreements
Pledged Asset Carrying Value	245,094						245,094
Maximum Facility Size	201,238						201,238
Principal Amount	177,217						177,217		164,611
Secured financing agreements | Ireland Portfolio Mortgage
Secured Financing Agreements
Pledged Asset Carrying Value	491,298						491,298
Maximum Facility Size	344,525						344,525
Principal Amount	344,525						$ 344,525		309,246
Secured financing agreements | Ireland Portfolio Mortgage | EURIBOR
Secured Financing Agreements
Pricing margin (as a percent)							1.69%
Secured financing agreements | Woodstar Portfolio Mortgages
Secured Financing Agreements
Pledged Asset Carrying Value	369,519						$ 369,519
Maximum Facility Size	276,748						276,748
Principal Amount	$ 276,748						$ 276,748		276,748
Secured financing agreements | Woodstar Portfolio Mortgages | Minimum
Secured Financing Agreements
Interest rate (as a percent)	3.72%						3.72%
Secured financing agreements | Woodstar Portfolio Mortgages | Maximum
Secured Financing Agreements
Interest rate (as a percent)	3.97%						3.97%
Secured financing agreements | Woodstar Portfolio Government Financing
Secured Financing Agreements
Pledged Asset Carrying Value	$ 308,805						$ 308,805
Maximum Facility Size	133,967						133,967
Principal Amount	$ 133,967						$ 133,967		135,584
Secured financing agreements | Woodstar Portfolio Government Financing | Minimum
Secured Financing Agreements
Interest rate (as a percent)	1.00%						1.00%
Secured financing agreements | Woodstar Portfolio Government Financing | Maximum
Secured Financing Agreements
Interest rate (as a percent)	5.00%						5.00%
Secured financing agreements | Medical Office Portfolio Mortgages
Secured Financing Agreements
Pledged Asset Carrying Value	$ 741,304						$ 741,304
Maximum Facility Size	527,124						527,124
Principal Amount	497,613						$ 497,613		491,197
Secured financing agreements | Medical Office Portfolio Mortgages | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.50%
Secured financing agreements | Term Loan A
Secured Financing Agreements
Pledged Asset Carrying Value	992,366						$ 992,366
Maximum Facility Size	300,000						300,000
Principal Amount	300,000						$ 300,000		$ 300,000
Secured financing agreements | Term Loan A | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.25%
Secured financing agreements | Revolving Secured Financing
Secured Financing Agreements
Maximum Facility Size	100,000						$ 100,000
Secured financing agreements | Revolving Secured Financing | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							2.25%
Secured financing agreements | FHLB
Secured Financing Agreements
Pledged Asset Carrying Value	338,956						$ 338,956
Maximum Facility Size	250,000						250,000
Principal Amount	$ 250,000						$ 250,000
Secured financing agreements | FHLB | Minimum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							0.15%
Secured financing agreements | FHLB | Maximum | LIBOR
Secured Financing Agreements
Pricing margin (as a percent)							0.34%