Segment Reporting - Summary Information for Reportable Segments (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	[1]
Segment Reporting [Abstract]
Number of operating segments | segment				3
Segment Reporting Information [Line Items]
Rental income		$ 412,147		$ 1,178,817
Rental income			$ 342,887		$ 1,019,857
Total revenues		1,266,133	1,236,379	3,858,484	3,459,256
Property operating expenses		655,588	657,157	2,027,522	1,782,373
Consolidated net operating income		610,545	579,222	1,830,962	1,676,883
Depreciation and amortization		272,445	243,149	764,429	707,625
Interest expense		137,343	138,032	423,911	382,223
General and administrative expenses		31,019	28,746	100,042	95,282
Loss (gain) on derivatives and financial instruments, net		1,244	8,991	670	(5,642)
Loss (gain) on extinguishment of debt, net		65,824	4,038	81,543	16,044
Provision for loan losses	$ 18,690	0	0	18,690	0
Impairment of assets		18,096	6,740	28,035	39,557
Other expenses		6,186	88,626	36,570	102,396
Income (loss) from continuing operations before income taxes and other items		78,388	60,900	377,072	339,398
Income tax (expense) benefit		(3,968)	(1,741)	(7,789)	(7,170)
(Loss) income from unconsolidated entities		3,262	344	(14,986)	(836)
Gain (loss) on real estate dispositions, net		570,250	24,723	735,977	373,662
Income (loss) from continuing operations		647,932	84,226	1,090,274	705,054
Net income (loss)		647,932	84,226	1,090,274	705,054
Total assets		31,863,955		31,863,955		$ 30,342,072
Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer		834,121	875,171	2,616,491	2,374,450
Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer		15,637	14,622	48,112	42,732
Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		4,228	3,699	15,064	22,217
Operating Segments | Seniors Housing Operating
Segment Reporting Information [Line Items]
Rental income		0		0
Rental income			0		0
Total revenues		835,496	876,505	2,623,411	2,378,839
Property operating expenses		581,341	610,659	1,826,344	1,648,262
Consolidated net operating income		254,155	265,846	797,067	730,577
Depreciation and amortization		148,126	136,532	416,252	397,080
Interest expense		16,356	17,319	52,179	51,225
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		1,450	0	1,450	110
Provision for loan losses				0
Impairment of assets		2,599	562	2,599	5,075
Other expenses		4,274	(811)	19,077	5,168
Income (loss) from continuing operations before income taxes and other items		81,350	112,244	305,510	271,919
Income tax (expense) benefit		(2,554)	211	(2,798)	(2,244)
(Loss) income from unconsolidated entities		(3,859)	(6,705)	(37,892)	(21,389)
Gain (loss) on real estate dispositions, net		519,203	(1)	518,493	3
Income (loss) from continuing operations		594,140	105,749	783,313	248,289
Net income (loss)		594,140	105,749	783,313	248,289
Total assets		15,095,737		15,095,737
Operating Segments | Seniors Housing Operating | Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer		834,121	875,171	2,616,491	2,374,450
Operating Segments | Seniors Housing Operating | Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer		0	159	0	416
Operating Segments | Seniors Housing Operating | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		1,375	1,175	6,920	3,973
Operating Segments | Triple-net
Segment Reporting Information [Line Items]
Rental income		227,499		681,893
Rental income			203,039		607,831
Total revenues		244,607	219,110	733,606	666,289
Property operating expenses		13,922	426	41,700	583
Consolidated net operating income		230,685	218,684	691,906	665,706
Depreciation and amortization		57,147	60,383	174,551	171,724
Interest expense		3,076	3,500	9,741	10,742
General and administrative expenses		0	0	0	0
Loss (gain) on derivatives and financial instruments, net		1,244	8,991	670	(5,642)
Loss (gain) on extinguishment of debt, net		0	0	0	(32)
Provision for loan losses				18,690
Impairment of assets		12,314	6,178	11,374	34,482
Other expenses		(2,496)	87,076	6,093	89,153
Income (loss) from continuing operations before income taxes and other items		159,400	52,556	470,787	365,279
Income tax (expense) benefit		12	1,116	(2,300)	(708)
(Loss) income from unconsolidated entities		5,276	5,377	17,512	16,260
Gain (loss) on real estate dispositions, net		51,529	24,782	217,973	158,938
Income (loss) from continuing operations		216,217	83,831	703,972	539,769
Net income (loss)		216,217	83,831	703,972	539,769
Total assets		9,350,606		9,350,606
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		15,279	14,378	47,343	42,176
Operating Segments | Triple-net | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		1,829	1,693	4,370	16,282
Operating Segments | Outpatient Medical
Segment Reporting Information [Line Items]
Rental income		184,648		496,924
Rental income			139,848		412,026
Total revenues		185,189	140,069	498,015	412,567
Property operating expenses		60,325	46,072	159,478	133,528
Consolidated net operating income		124,864	93,997	338,537	279,039
Depreciation and amortization		67,172	46,234	173,626	138,821
Interest expense		3,363	1,643	10,097	4,975
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	0	11,928
Provision for loan losses				0
Impairment of assets		3,183	0	14,062	0
Other expenses		524	1,055	1,274	3,748
Income (loss) from continuing operations before income taxes and other items		50,622	45,065	139,478	119,567
Income tax (expense) benefit		(302)	239	(1,253)	(567)
(Loss) income from unconsolidated entities		1,845	1,672	5,394	4,293
Gain (loss) on real estate dispositions, net		(482)	(58)	(489)	214,721
Income (loss) from continuing operations		51,683	46,918	143,130	338,014
Net income (loss)		51,683	46,918	143,130	338,014
Total assets		7,173,763		7,173,763
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		358	85	769	140
Operating Segments | Outpatient Medical | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer		183	136	322	401
Non-segment / Corporate
Segment Reporting Information [Line Items]
Rental income		0		0
Rental income			0		0
Total revenues		841	695	3,452	1,561
Property operating expenses		0	0	0	0
Consolidated net operating income		841	695	3,452	1,561
Depreciation and amortization		0	0	0	0
Interest expense		114,548	115,570	351,894	315,281
General and administrative expenses		31,019	28,746	100,042	95,282
Loss (gain) on derivatives and financial instruments, net		0	0	0	0
Loss (gain) on extinguishment of debt, net		64,374	4,038	80,093	4,038
Provision for loan losses				0
Impairment of assets		0	0	0	0
Other expenses		3,884	1,306	10,126	4,327
Income (loss) from continuing operations before income taxes and other items		(212,984)	(148,965)	(538,703)	(417,367)
Income tax (expense) benefit		(1,124)	(3,307)	(1,438)	(3,651)
(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		(214,108)	(152,272)	(540,141)	(421,018)
Net income (loss)		(214,108)	(152,272)	(540,141)	(421,018)
Total assets		243,849		243,849
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		$ 841	$ 695	$ 3,452	$ 1,561

[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.