Segment Reporting - Summary Information for Reportable Segments (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) segment	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	[1]
Segment Reporting [Abstract]
Number of operating segments | segment				3
Segment Reporting Information [Line Items]
Rental income		$ 385,586		$ 766,670
Rental income			$ 333,601		$ 676,970
Total revenues		1,320,106	1,125,912	2,592,351	2,222,877
Property operating expenses		701,127	568,751	1,371,934	1,125,216
Consolidated net operating income		618,979	557,161	1,220,417	1,097,661
Depreciation and amortization		248,052	236,275	491,984	464,476
Interest expense		141,336	121,416	286,568	244,191
General and administrative expenses		33,741	32,831	69,023	66,536
Loss (gain) on derivatives and financial instruments, net		1,913	(7,460)	(574)	(14,633)
Loss (gain) on extinguishment of debt, net		0	299	15,719	12,006
Provision for loan losses	$ 18,690	0	0	18,690	0
Impairment of assets		9,939	4,632	9,939	32,817
Other expenses		21,628	10,058	30,384	13,770
Income (loss) from continuing operations before income taxes and other items		162,370	159,110	298,684	278,498
Income tax (expense) benefit		(1,599)	(3,841)	(3,821)	(5,429)
(Loss) income from unconsolidated entities		(9,049)	1,249	(18,248)	(1,180)
Gain (loss) on real estate dispositions, net		(1,682)	10,755	165,727	348,939
Income (loss) from continuing operations		150,040	167,273	442,342	620,828
Net income (loss)		150,040	167,273	442,342	620,828
Total assets		33,148,697		33,148,697		$ 30,342,072
Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer		914,085	763,345	1,782,370	1,499,279
Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer		17,356	13,462	32,475	28,110
Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		3,079	15,504	10,836	18,518
Operating Segments | Seniors Housing Operating
Segment Reporting Information [Line Items]
Rental income		0		0
Rental income			0		0
Total revenues		915,529	765,167	1,787,915	1,502,334
Property operating expenses		637,317	525,662	1,245,003	1,037,603
Consolidated net operating income		278,212	239,505	542,912	464,731
Depreciation and amortization		136,551	134,779	268,126	260,548
Interest expense		17,572	16,971	35,823	33,906
General and administrative expenses		0	0	0	0
Loss (gain) on derivatives and financial instruments, net		0	0	0	0
Loss (gain) on extinguishment of debt, net			299	0	110
Provision for loan losses				0
Impairment of assets		0	2,212	0	4,513
Other expenses		11,857	6,167	14,803	5,979
Income (loss) from continuing operations before income taxes and other items		112,232	79,077	224,160	159,675
Income tax (expense) benefit		375	(2,617)	(244)	(2,455)
(Loss) income from unconsolidated entities		(17,453)	(5,204)	(34,033)	(14,684)
Gain (loss) on real estate dispositions, net		(550)	(1)	(710)	4
Income (loss) from continuing operations		94,604	71,255	189,173	142,540
Net income (loss)		94,604	71,255	189,173	142,540
Total assets		16,440,104		16,440,104
Operating Segments | Seniors Housing Operating | Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer		914,085	763,345	1,782,370	1,499,279
Operating Segments | Seniors Housing Operating | Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer		0	172	0	257
Operating Segments | Seniors Housing Operating | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		1,444	1,650	5,545	2,798
Operating Segments | Triple-net
Segment Reporting Information [Line Items]
Rental income		222,362		454,394
Rental income			197,961		404,792
Total revenues		240,758	224,420	488,999	447,179
Property operating expenses		12,823	136	27,778	157
Consolidated net operating income		227,935	224,284	461,221	447,022
Depreciation and amortization		56,056	55,309	117,404	111,341
Interest expense		3,225	3,800	6,665	7,242
General and administrative expenses		0	0	0	0
Loss (gain) on derivatives and financial instruments, net		1,913	(7,460)	(574)	(14,633)
Loss (gain) on extinguishment of debt, net			0	0	(32)
Provision for loan losses				18,690
Impairment of assets		(940)	2,420	(940)	28,304
Other expenses		5,560	957	8,589	2,077
Income (loss) from continuing operations before income taxes and other items		162,121	169,258	311,387	312,723
Income tax (expense) benefit		(1,361)	(688)	(2,312)	(1,824)
(Loss) income from unconsolidated entities		6,578	5,062	12,236	10,883
Gain (loss) on real estate dispositions, net		(1,130)	10,759	166,444	134,156
Income (loss) from continuing operations		166,208	184,391	487,755	455,938
Net income (loss)		166,208	184,391	487,755	455,938
Total assets		9,494,388		9,494,388
Operating Segments | Triple-net | Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer		0	0	0	0
Operating Segments | Triple-net | Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer		17,118	13,247	32,064	27,798
Operating Segments | Triple-net | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		1,278	13,212	2,541	14,589
Operating Segments | Outpatient Medical
Segment Reporting Information [Line Items]
Rental income		163,224		312,276
Rental income			135,640		272,178
Total revenues		163,365	135,827	312,826	272,498
Property operating expenses		50,987	42,953	99,153	87,456
Consolidated net operating income		112,378	92,874	213,673	185,042
Depreciation and amortization		55,445	46,187	106,454	92,587
Interest expense		3,386	1,656	6,734	3,332
General and administrative expenses		0	0	0	0
Loss (gain) on derivatives and financial instruments, net		0	0	0	0
Loss (gain) on extinguishment of debt, net			0	0	11,928
Provision for loan losses				0
Impairment of assets		10,879	0	10,879	0
Other expenses		(4)	2,095	750	2,693
Income (loss) from continuing operations before income taxes and other items		42,672	42,936	88,856	74,502
Income tax (expense) benefit		(586)	(378)	(951)	(806)
(Loss) income from unconsolidated entities		1,826	1,391	3,549	2,621
Gain (loss) on real estate dispositions, net		(2)	(3)	(7)	214,779
Income (loss) from continuing operations		43,910	43,946	91,447	291,096
Net income (loss)		43,910	43,946	91,447	291,096
Total assets		7,004,561		7,004,561
Operating Segments | Outpatient Medical | Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer		0	0	0	0
Operating Segments | Outpatient Medical | Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer		238	43	411	55
Operating Segments | Outpatient Medical | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		(97)	144	139	265
Non-segment / Corporate
Segment Reporting Information [Line Items]
Rental income		0		0
Rental income			0		0
Total revenues		454	498	2,611	866
Property operating expenses		0	0	0	0
Consolidated net operating income		454	498	2,611	866
Depreciation and amortization		0	0	0	0
Interest expense		117,153	98,989	237,346	199,711
General and administrative expenses		33,741	32,831	69,023	66,536
Loss (gain) on derivatives and financial instruments, net		0	0	0	0
Loss (gain) on extinguishment of debt, net			0	15,719	0
Provision for loan losses				0
Impairment of assets		0	0	0	0
Other expenses		4,215	839	6,242	3,021
Income (loss) from continuing operations before income taxes and other items		(154,655)	(132,161)	(325,719)	(268,402)
Income tax (expense) benefit		(27)	(158)	(314)	(344)
(Loss) income from unconsolidated entities		0	0	0	0
Gain (loss) on real estate dispositions, net		0	0	0	0
Income (loss) from continuing operations		(154,682)	(132,319)	(326,033)	(268,746)
Net income (loss)		(154,682)	(132,319)	(326,033)	(268,746)
Total assets		209,644		209,644
Non-segment / Corporate | Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer		0	0	0	0
Non-segment / Corporate | Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer		0	0	0	0
Non-segment / Corporate | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		$ 454	$ 498	$ 2,611	$ 866

[1] The consolidated balance sheet at December 31, 2018 has been derived from the audited financial statements at that date but does not include all of the information and footnotes required by U.S. generally accepted accounting principles for complete financial statements.