Acquisitions and Dispositions (Details) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended		24 Months Ended
	Sep. 25, 2017 USD ($) ft² item	Sep. 30, 2017 USD ($) item	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) item	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) ft² item	Dec. 31, 2015 USD ($) ft² property item	Dec. 31, 2016 USD ($) ft² item
Acquisitions
Properties, net		$ 2,521,342		$ 2,521,342		$ 1,944,720		$ 1,944,720
Liabilities assumed:
Revenues		226,767	$ 204,705	637,056	$ 600,190
Net income (loss)		92,799	105,813	318,886	244,930
Bargain purchase gains					8,406
Depreciation and amortization				64,937	49,081
Gain (loss) on derivative financial instruments, net		(24,224)	$ (2,328)	(66,159)	(6,793)
Goodwill		$ 140,437		$ 140,437		140,437		$ 140,437
Master Lease Portfolio
Acquisitions
Number of retail properties acquired | item	20
Number of industrial properties acquired | item	3
Purchase price	$ 553,300
Number of square feet of properties | ft²	5,300,000
Term of master lease agreements	24 years 7 months 6 days
Maximum borrowing capacity	$ 265,900
Acquisition-related costs	$ 3,700
Master Lease Portfolio | Geographic Concentration Risk | Utah, Florida, Texas and Minnesota | Minimum
Acquisitions
Concentration risk (as a percent)	50.00%
Medical Office Portfolio
Acquisitions
Purchase price						$ 758,800
Area of property | ft²						1,900,000		1,900,000
Number of acquired properties closed | item						34
Liabilities assumed:
Bargain purchase gains						$ 0
Goodwill						$ 0		$ 0
Woodstar Portfolio
Acquisitions
Purchase price								$ 421,500
Number of properties in portfolio investment | item		32		32
Number of acquired properties closed | item						14	18	32
Number of units acquired | item				8,948
Liabilities assumed:
Bargain purchase gains						$ 8,400
Goodwill						0		$ 0
Ireland Portfolio
Acquisitions
Area of property | ft²							600,000
Debt incurred to fund acquisition							$ 328,600
Number of properties sold | item				1
Proceeds from sale of property				$ 3,900
Assets acquired:
Total assets acquired							518,200
Liabilities assumed:
Total liabilities assumed							283,000
Bargain purchase gains							0
Goodwill							$ 0
REIS Equity Portfolio
Assets acquired:
Properties		$ 34,902		34,902
Intangible assets		4,272		4,272
Other assets		1		1
Total assets acquired		39,175		39,175
Liabilities assumed:
Accounts payable, accrued expenses and other liabilities		1,329		1,329
Total liabilities assumed		1,329		1,329
Net assets acquired		37,846		37,846
REIS Equity Portfolio | ASU 2015-16
Assets acquired:
Properties		12,747		12,747
Intangible assets		3,468		3,468
Other assets		97		97
Total assets acquired		16,312		16,312
Liabilities assumed:
Accounts payable, accrued expenses and other liabilities		1,397		1,397
Total liabilities assumed		1,397		1,397
Non-controlling interests		3,084		3,084
Net assets acquired		11,831		11,831
REIS Equity Portfolio | ASU 2015-16 | Initial Amount
Assets acquired:
Properties		12,087		12,087
Intangible assets		4,270		4,270
Other assets		97		97
Total assets acquired		16,454		16,454
Liabilities assumed:
Accounts payable, accrued expenses and other liabilities		1,539		1,539
Total liabilities assumed		1,539		1,539
Non-controlling interests		3,084		3,084
Net assets acquired		11,831		11,831
REIS Equity Portfolio | ASU 2015-16 | Measurement Period Adjustments
Assets acquired:
Properties		660		660
Intangible assets		(802)		(802)
Total assets acquired		(142)		(142)
Liabilities assumed:
Accounts payable, accrued expenses and other liabilities		(142)		(142)
Total liabilities assumed		$ (142)		(142)
Net Leased Office Property | Ireland Portfolio
Acquisitions
Number of properties in portfolio investment | property							12
Multifamily Property | Ireland Portfolio
Acquisitions
Number of properties in portfolio investment | property							1
Non-Controlling Interests
Liabilities assumed:
Net income (loss)				$ 10,720	$ 1,034
Investment and Servicing Segment
Acquisitions
Purchase price								$ 268,500
Number of real estate business acquired | item		1		2				24
Aggregate gross acquisition price		$ 18,200		$ 37,200
Number of properties sold | item		2		4
Proceeds from sale of property		$ 26,000		$ 40,700
Gain on sale of property		11,200		16,300
Liabilities assumed:
Bargain purchase gains				600		$ 8,800
Goodwill		0		0
Investment and Servicing Segment | Non-Controlling Interests
Acquisitions
Gain on sale of property		$ 2,400		$ 2,400