Segment Reporting - Summary Information for Reportable Segments (Details) $ in Thousands	1 Months Ended		3 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	[1]
Segment Reporting [Abstract]
Number of operating segments | segment			3
Segment Reporting Information [Line Items]
Rental income			$ 389,960	$ 381,084
Total revenues			1,258,602	1,272,245
Property operating expenses			681,781	670,807
Consolidated net operating income			576,821	601,438
Depreciation and amortization			274,801	243,932
Interest expense			142,007	145,232
General and administrative expenses			35,481	35,282
Loss (gain) on derivatives and financial instruments, net			7,651	(2,487)
Loss (gain) on extinguishment of debt, net			0	15,719
Provision for loan losses	$ 6,898	$ 18,690	7,072	18,690
Impairment of assets			27,827	0
Other expenses			6,292	8,756
Income (loss) from continuing operations before income taxes and other items			75,690	136,314
Income tax (expense) benefit			(5,442)	(2,222)
Income (loss) from unconsolidated entities			(3,692)	(9,199)
Gain (loss) on real estate dispositions, net			262,824	167,409
Income (loss) from continuing operations			329,380	292,302
Net income (loss)			329,380	292,302
Total assets	32,923,028		32,923,028		$ 33,380,751
Straight line rent adjustment			32,268
Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer			849,972	868,285
Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer			15,241	15,119
Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer			3,429	7,757
Operating Segments | Seniors Housing Operating
Segment Reporting Information [Line Items]
Rental income			0	0
Total revenues			851,128	872,386
Property operating expenses			607,871	607,686
Consolidated net operating income			243,257	264,700
Depreciation and amortization			146,774	131,575
Interest expense			16,434	18,251
General and administrative expenses			0	0
Loss (gain) on derivatives and financial instruments, net			0	0
Loss (gain) on extinguishment of debt, net				0
Provision for loan losses			0	0
Impairment of assets			3,495
Other expenses			2,989	2,946
Income (loss) from continuing operations before income taxes and other items			73,565	111,928
Income tax (expense) benefit			0	0
Income (loss) from unconsolidated entities			(11,024)	(16,580)
Gain (loss) on real estate dispositions, net			(149)	(160)
Income (loss) from continuing operations			62,392	95,188
Net income (loss)			62,392	95,188
Total assets	15,691,090		15,691,090
Operating Segments | Seniors Housing Operating | Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer			849,972	868,285
Operating Segments | Seniors Housing Operating | Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer			104	0
Operating Segments | Seniors Housing Operating | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer			1,052	4,101
Operating Segments | Triple-net
Segment Reporting Information [Line Items]
Rental income			191,385	232,032
Total revenues			207,729	248,241
Property operating expenses			13,302	14,955
Consolidated net operating income			194,427	233,286
Depreciation and amortization			57,694	61,348
Interest expense			2,852	3,440
General and administrative expenses			0	0
Loss (gain) on derivatives and financial instruments, net			7,651	(2,487)
Loss (gain) on extinguishment of debt, net				0
Provision for loan losses			7,072	18,690
Impairment of assets			24,332
Other expenses			513	3,029
Income (loss) from continuing operations before income taxes and other items			94,313	149,266
Income tax (expense) benefit			0	0
Income (loss) from unconsolidated entities			5,796	5,658
Gain (loss) on real estate dispositions, net			49,637	167,574
Income (loss) from continuing operations			149,746	322,498
Net income (loss)			149,746	322,498
Total assets	9,200,458		9,200,458
Operating Segments | Triple-net | Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer			0	0
Operating Segments | Triple-net | Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer			14,671	14,946
Operating Segments | Triple-net | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer			1,673	1,263
Operating Segments | Outpatient Medical
Segment Reporting Information [Line Items]
Rental income			198,575	149,052
Total revenues			199,329	149,461
Property operating expenses			60,608	48,166
Consolidated net operating income			138,721	101,295
Depreciation and amortization			70,333	51,009
Interest expense			4,808	3,348
General and administrative expenses			0	0
Loss (gain) on derivatives and financial instruments, net			0	0
Loss (gain) on extinguishment of debt, net				0
Provision for loan losses			0	0
Impairment of assets			0
Other expenses			1,007	754
Income (loss) from continuing operations before income taxes and other items			62,573	46,184
Income tax (expense) benefit			0	0
Income (loss) from unconsolidated entities			1,536	1,723
Gain (loss) on real estate dispositions, net			213,336	(5)
Income (loss) from continuing operations			277,445	47,902
Net income (loss)			277,445	47,902
Total assets	7,722,679		7,722,679
Operating Segments | Outpatient Medical | Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer			0	0
Operating Segments | Outpatient Medical | Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer			466	173
Operating Segments | Outpatient Medical | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer			288	236
Non-segment / Corporate
Segment Reporting Information [Line Items]
Rental income			0	0
Total revenues			416	2,157
Property operating expenses			0	0
Consolidated net operating income			416	2,157
Depreciation and amortization			0	0
Interest expense			117,913	120,193
General and administrative expenses			35,481	35,282
Loss (gain) on derivatives and financial instruments, net			0	0
Loss (gain) on extinguishment of debt, net				15,719
Provision for loan losses			0	0
Impairment of assets			0
Other expenses			1,783	2,027
Income (loss) from continuing operations before income taxes and other items			(154,761)	(171,064)
Income tax (expense) benefit			5,442	2,222
Income (loss) from unconsolidated entities			0	0
Gain (loss) on real estate dispositions, net			0	0
Income (loss) from continuing operations			(160,203)	(173,286)
Net income (loss)			(160,203)	(173,286)
Total assets	$ 308,801		308,801
Non-segment / Corporate | Resident fees and services
Segment Reporting Information [Line Items]
Revenue from contract with customer			0	0
Non-segment / Corporate | Interest income
Segment Reporting Information [Line Items]
Revenue from contract with customer			0	0
Non-segment / Corporate | Other income
Segment Reporting Information [Line Items]
Revenue from contract with customer			$ 416	$ 2,157

[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.