Segment Reporting - Summary Information for Reportable Segments (Details) $ in Thousands	1 Months Ended	3 Months Ended			6 Months Ended
	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	[1]
Segment Reporting [Abstract]
Number of operating segments | segment					3
Segment Reporting Information [Line Items]
Rental income		$ 396,305		$ 385,586	$ 786,265	$ 766,670
Total revenues		1,188,475		1,320,106	2,447,077	2,592,351
Property operating expenses		660,764		701,127	1,342,545	1,371,934
Consolidated net operating income		527,711		618,979	1,104,532	1,220,417
Depreciation and amortization		265,371		248,052	540,172	491,984
Interest expense		126,357		141,336	268,364	286,568
General and administrative expenses		34,062		33,741	69,543	69,023
Loss (gain) on derivatives and financial instruments, net		1,434		1,913	9,085	(574)
Loss (gain) on extinguishment of debt, net		249		0	249	15,719
Provision for loan losses	$ 18,690	1,422		0	8,494	18,690
Impairment of assets		75,151		9,939	102,978	9,939
Other expenses		19,411		21,628	25,703	30,384
Income (loss) from continuing operations before income taxes and other items		4,254		162,370	79,944	298,684
Income tax (expense) benefit		(2,233)		(1,599)	(7,675)	(3,821)
Income (loss) from unconsolidated entities		1,332		(9,049)	(2,360)	(18,248)
Gain (loss) on real estate dispositions, net		155,863		(1,682)	418,687	165,727
Income (loss) from continuing operations		159,216		150,040	488,596	442,342
Net income (loss)		159,216		150,040	488,596	442,342
Total assets		33,162,652			33,162,652		$ 33,380,751
Straight line rent adjustment			$ 32,268
Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer		769,560		914,085	1,619,532	1,782,370
Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer		16,069		17,356	31,310	32,475
Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		6,541		3,079	9,970	10,836
Operating Segments | Seniors Housing Operating
Segment Reporting Information [Line Items]
Rental income		0		0	0	0
Total revenues		773,650		915,529	1,624,778	1,787,915
Property operating expenses		595,513		637,317	1,203,384	1,245,003
Consolidated net operating income		178,137		278,212	421,394	542,912
Depreciation and amortization		139,163		136,551	285,937	268,126
Interest expense		14,029		17,572	30,463	35,823
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		(492)			(492)	0
Provision for loan losses		0			0	0
Impairment of assets		75,151		0	78,646	0
Other expenses		5,251		11,857	8,240	14,803
Income (loss) from continuing operations before income taxes and other items		(54,965)		112,232	18,600	224,160
Income tax (expense) benefit		0		0	0	0
Income (loss) from unconsolidated entities		(6,787)		(17,453)	(17,811)	(34,033)
Gain (loss) on real estate dispositions, net		14,465		(550)	14,316	(710)
Income (loss) from continuing operations		(47,287)		94,229	15,105	189,417
Net income (loss)		(47,287)		94,229	15,105	189,417
Total assets		14,915,588			14,915,588
Operating Segments | Seniors Housing Operating | Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer		769,560		914,085	1,619,532	1,782,370
Operating Segments | Seniors Housing Operating | Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer		88		0	192	0
Operating Segments | Seniors Housing Operating | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		4,002		1,444	5,054	5,545
Operating Segments | Triple-net
Segment Reporting Information [Line Items]
Rental income		217,492		222,362	408,877	454,394
Total revenues		233,619		240,758	441,348	488,999
Property operating expenses		13,563		12,823	26,865	27,778
Consolidated net operating income		220,056		227,935	414,483	461,221
Depreciation and amortization		58,138		56,056	115,832	117,404
Interest expense		2,746		3,225	5,598	6,665
General and administrative expenses		0		0	0	0
Loss (gain) on derivatives and financial instruments, net		1,434		1,913	9,085	(574)
Loss (gain) on extinguishment of debt, net		0			0	0
Provision for loan losses		1,451			8,523	18,690
Impairment of assets		0		(940)	24,332	(940)
Other expenses		3,500		5,560	4,013	8,589
Income (loss) from continuing operations before income taxes and other items		152,787		162,121	247,100	311,387
Income tax (expense) benefit		0		0	0	0
Income (loss) from unconsolidated entities		6,403		6,578	12,199	12,236
Gain (loss) on real estate dispositions, net		2,148		(1,130)	51,785	166,444
Income (loss) from continuing operations		161,338		167,569	311,084	490,067
Net income (loss)		161,338		167,569	311,084	490,067
Total assets		9,189,707			9,189,707
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		15,520		17,118	30,191	32,064
Operating Segments | Triple-net | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		607		1,278	2,280	2,541
Operating Segments | Outpatient Medical
Segment Reporting Information [Line Items]
Rental income		178,813		163,224	377,388	312,276
Total revenues		180,831		163,365	380,160	312,826
Property operating expenses		51,688		50,987	112,296	99,153
Consolidated net operating income		129,143		112,378	267,864	213,673
Depreciation and amortization		68,070		55,445	138,403	106,454
Interest expense		4,326		3,386	9,134	6,734
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		741			741	0
Provision for loan losses		(29)			(29)	0
Impairment of assets		0		10,879	0	10,879
Other expenses		6,456		(4)	7,463	750
Income (loss) from continuing operations before income taxes and other items		49,579		42,672	112,152	88,856
Income tax (expense) benefit		0		0	0	0
Income (loss) from unconsolidated entities		1,716		1,826	3,252	3,549
Gain (loss) on real estate dispositions, net		139,250		(2)	352,586	(7)
Income (loss) from continuing operations		190,545		44,496	467,990	92,398
Net income (loss)		190,545		44,496	467,990	92,398
Total assets		7,434,364			7,434,364
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		461		238	927	411
Operating Segments | Outpatient Medical | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		1,557		(97)	1,845	139
Non-segment / Corporate
Segment Reporting Information [Line Items]
Rental income		0		0	0	0
Total revenues		375		454	791	2,611
Property operating expenses		0		0	0	0
Consolidated net operating income		375		454	791	2,611
Depreciation and amortization		0		0	0	0
Interest expense		105,256		117,153	223,169	237,346
General and administrative expenses		34,062		33,741	69,543	69,023
Loss (gain) on derivatives and financial instruments, net		0		0	0	0
Loss (gain) on extinguishment of debt, net		0			0	15,719
Provision for loan losses		0			0	0
Impairment of assets		0		0	0	0
Other expenses		4,204		4,215	5,987	6,242
Income (loss) from continuing operations before income taxes and other items		(143,147)		(154,655)	(297,908)	(325,719)
Income tax (expense) benefit		(2,233)		(1,599)	(7,675)	(3,821)
Income (loss) from unconsolidated entities		0		0	0	0
Gain (loss) on real estate dispositions, net		0		0	0	0
Income (loss) from continuing operations		(145,380)		(156,254)	(305,583)	(329,540)
Net income (loss)		(145,380)		(156,254)	(305,583)	(329,540)
Total assets		1,622,993			1,622,993
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		$ 375		$ 454	$ 791	$ 2,611

[1] The consolidated balance sheet at December 31, 2019 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.