Segment Reporting - Summary Information for Reportable Segments (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting [Abstract]
Number of operating segments | segment				3
Segment Reporting Information [Line Items]
Rental income		$ 275,046	$ 412,147	$ 1,061,311	$ 1,178,817
Total revenues		1,036,874	1,266,133	3,483,951	3,858,484
Property operating expenses		634,717	655,588	1,977,262	2,027,522
Consolidated net operating income		402,157	610,545	1,506,689	1,830,962
Depreciation and amortization		255,532	272,445	795,704	764,429
Interest expense		124,851	137,343	393,215	423,911
General and administrative expenses		31,003	31,019	100,546	100,042
Loss (gain) on derivatives and financial instruments, net		1,395	1,244	10,480	670
Loss (gain) on extinguishment of debt, net		33,004	65,824	33,253	81,543
Provision for loan losses	$ 18,690	2,857	0	11,351	18,690
Impairment of assets		23,313	18,096	126,291	28,035
Other expenses		11,544	6,186	37,247	36,570
Income (loss) from continuing operations before income taxes and other items		(81,342)	78,388	(1,398)	377,072
Income tax (expense) benefit		(2,003)	(3,968)	(9,678)	(7,789)
Income (loss) from unconsolidated entities		(5,981)	3,262	(8,341)	(14,986)
Gain (loss) on real estate dispositions, net		484,304	570,250	902,991	735,977
Income (loss) from continuing operations		394,978	647,932	883,574	1,090,274
Net income (loss)		394,978	647,932	883,574	1,090,274
Total assets		32,541,597		32,541,597		$ 33,380,751
Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer		740,956	834,121	2,360,488	2,616,491
Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer		16,750	15,637	48,060	48,112
Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		4,122	4,228	14,092	15,064
Operating Segments | Seniors Housing Operating
Segment Reporting Information [Line Items]
Rental income		0	0	0	0
Total revenues		742,065	835,496	2,366,843	2,623,411
Property operating expenses		567,704	581,341	1,771,088	1,826,344
Consolidated net operating income		174,361	254,155	595,755	797,067
Depreciation and amortization		133,224	148,126	419,161	416,252
Interest expense		12,728	16,356	43,191	52,179
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	1,450	(492)	1,450
Provision for loan losses		86		86	0
Impairment of assets		12,778	2,599	91,424	2,599
Other expenses		6,488	4,274	13,463	19,077
Income (loss) from continuing operations before income taxes and other items		9,057	81,350	28,922	305,510
Income tax (expense) benefit		0	0	0	0
Income (loss) from unconsolidated entities		(7,678)	(3,859)	(25,489)	(37,892)
Gain (loss) on real estate dispositions, net		313,319	519,203	327,635	518,493
Income (loss) from continuing operations		314,698	596,694	331,068	786,111
Net income (loss)		314,698	596,694	331,068	786,111
Total assets		15,063,405		15,063,405
Operating Segments | Seniors Housing Operating | Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer		740,956	834,121	2,360,488	2,616,491
Operating Segments | Seniors Housing Operating | Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer		113	0	305	0
Operating Segments | Seniors Housing Operating | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		996	1,375	6,050	6,920
Operating Segments | Triple-net
Segment Reporting Information [Line Items]
Rental income		103,938	227,499	512,815	681,893
Total revenues		120,928	244,607	562,276	733,606
Property operating expenses		12,567	13,922	39,432	41,700
Consolidated net operating income		108,361	230,685	522,844	691,906
Depreciation and amortization		58,157	57,147	173,989	174,551
Interest expense		2,070	3,076	7,668	9,741
General and administrative expenses		0	0	0	0
Loss (gain) on derivatives and financial instruments, net		1,395	1,244	10,480	670
Loss (gain) on extinguishment of debt, net		0	0	0	0
Provision for loan losses		372		8,895	18,690
Impairment of assets		10,535	12,314	34,867	11,374
Other expenses		2,805	(2,496)	6,818	6,093
Income (loss) from continuing operations before income taxes and other items		33,027	159,400	280,127	470,787
Income tax (expense) benefit		0	0	0	0
Income (loss) from unconsolidated entities		(423)	5,276	11,776	17,512
Gain (loss) on real estate dispositions, net		(619)	51,529	51,166	217,973
Income (loss) from continuing operations		31,985	216,205	343,069	706,272
Net income (loss)		31,985	216,205	343,069	706,272
Total assets		9,075,910		9,075,910
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		15,877	15,279	46,068	47,343
Operating Segments | Triple-net | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		1,113	1,829	3,393	4,370
Operating Segments | Outpatient Medical
Segment Reporting Information [Line Items]
Rental income		171,108	184,648	548,496	496,924
Total revenues		172,704	185,189	552,864	498,015
Property operating expenses		52,728	60,325	165,024	159,478
Consolidated net operating income		119,976	124,864	387,840	338,537
Depreciation and amortization		64,151	67,172	202,554	173,626
Interest expense		4,287	3,363	13,421	10,097
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	741	0
Provision for loan losses		2,399		2,370	0
Impairment of assets		0	3,183	0	14,062
Other expenses		781	524	8,244	1,274
Income (loss) from continuing operations before income taxes and other items		48,358	50,622	160,510	139,478
Income tax (expense) benefit		0	0	0	0
Income (loss) from unconsolidated entities		2,120	1,845	5,372	5,394
Gain (loss) on real estate dispositions, net		171,604	(482)	524,190	(489)
Income (loss) from continuing operations		222,082	51,985	690,072	144,383
Net income (loss)		222,082	51,985	690,072	144,383
Total assets		6,892,502		6,892,502
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		760	358	1,687	769
Operating Segments | Outpatient Medical | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		836	183	2,681	322
Non-segment / Corporate
Segment Reporting Information [Line Items]
Rental income		0	0	0	0
Total revenues		1,177	841	1,968	3,452
Property operating expenses		1,718	0	1,718	0
Consolidated net operating income		(541)	841	250	3,452
Depreciation and amortization		0	0	0	0
Interest expense		105,766	114,548	328,935	351,894
General and administrative expenses		31,003	31,019	100,546	100,042
Loss (gain) on derivatives and financial instruments, net		0	0	0	0
Loss (gain) on extinguishment of debt, net		33,004	64,374	33,004	80,093
Provision for loan losses		0		0	0
Impairment of assets		0	0	0	0
Other expenses		1,470	3,884	8,722	10,126
Income (loss) from continuing operations before income taxes and other items		(171,784)	(212,984)	(470,957)	(538,703)
Income tax (expense) benefit		(2,003)	(3,968)	(9,678)	(7,789)
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		(173,787)	(216,952)	(480,635)	(546,492)
Net income (loss)		(173,787)	(216,952)	(480,635)	(546,492)
Total assets		1,509,780		1,509,780
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		$ 1,177	$ 841	$ 1,968	$ 3,452