Segment Reporting (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Total revenues	$ 1,125,912,000	$ 1,058,602,000	$ 2,222,877,000	$ 2,120,900,000
Property operating expenses	568,751,000	501,855,000	1,125,216,000	1,012,024,000
Net Operating Income from Continuing Operations	557,161,000	556,747,000	1,097,661,000	1,108,876,000
Interest expense	121,416,000	116,231,000	244,191,000	234,827,000
Loss (gain) on derivatives and financial instruments, net	(7,460,000)	736,000	(14,633,000)	1,960,000
Depreciation and amortization	236,275,000	224,847,000	464,476,000	453,124,000
General and administrative	32,831,000	32,632,000	66,536,000	63,733,000
Loss (gain) on extinguishment of debt, net	299,000	5,515,000	12,006,000	36,870,000
Impairment of assets	4,632,000	13,631,000	32,817,000	24,662,000
Other expenses	10,058,000	6,339,000	13,770,000	18,014,000
Income (loss) from continuing operations before income taxes and income from unconsolidated entities	159,110,000	156,816,000	278,498,000	275,686,000
Income tax expense	(3,841,000)	8,448,000	(5,429,000)	6,203,000
(Loss) income from unconsolidated entities	1,249,000	(3,978,000)	(1,180,000)	(27,084,000)
Income from continuing operations	156,518,000	161,286,000	271,889,000	254,805,000
Gain (loss) on real estate dispositions, net	10,755,000	42,155,000	348,939,000	286,247,000
Net income	167,273,000	$ 203,441,000	620,828,000	$ 541,052,000
Total assets	$ 27,618,982,000		$ 27,618,982,000		$ 27,944,445,000
Percent Of Assets	1.000		1.000		1.000
Percent Of Revenues	1.000	1.000	1.000	1.000
United States
Segment Reporting Information [Line Items]
Total revenues	$ 895,734,000	$ 851,943,000	$ 1,759,523,000	$ 1,710,611,000
Total assets	$ 21,959,262,000		$ 21,959,262,000		$ 22,274,443,000
Percent Of Assets	0.795		0.795		0.797
Percent Of Revenues	0.795	0.805	0.791	0.807
United Kingdom
Segment Reporting Information [Line Items]
Total revenues	$ 112,031,000	$ 99,747,000	$ 228,556,000	$ 193,590,000
Total assets	$ 3,208,929,000		$ 3,208,929,000		$ 3,239,039,000
Percent Of Assets	0.116		0.116		0.116
Percent Of Revenues	0.100	0.094	0.103	0.091
Canada
Segment Reporting Information [Line Items]
Total revenues	$ 118,147,000	$ 106,912,000	$ 234,798,000	$ 216,699,000
Total assets	$ 2,450,791,000		$ 2,450,791,000		$ 2,430,963,000
Percent Of Assets	0.089		0.089		0.087
Percent Of Revenues	0.105	0.101	0.106	0.102
Triple Net
Segment Reporting Information [Line Items]
Lease termination fee income	$ 10,805,000
Operating Segments | Triple Net
Segment Reporting Information [Line Items]
Total revenues	224,420,000	$ 241,347,000	$ 447,179,000	$ 491,081,000
Property operating expenses	136,000	0	157,000	0
Net Operating Income from Continuing Operations	224,284,000	241,347,000	447,022,000	491,081,000
Interest expense	3,800,000	2,515,000	7,242,000	8,025,000
Loss (gain) on derivatives and financial instruments, net	(7,460,000)	736,000	(14,633,000)	1,960,000
Depreciation and amortization	55,309,000	60,171,000	111,341,000	119,781,000
General and administrative	0	0	0	0
Loss (gain) on extinguishment of debt, net	0	0	(32,000)	29,083,000
Impairment of assets	2,420,000	4,846,000	28,304,000	4,846,000
Other expenses	957,000	2,181,000	2,077,000	7,190,000
Income (loss) from continuing operations before income taxes and income from unconsolidated entities	169,258,000	170,898,000	312,723,000	320,196,000
Income tax expense	(688,000)	(1,471,000)	(1,824,000)	(2,271,000)
(Loss) income from unconsolidated entities	5,062,000	3,867,000	10,883,000	9,505,000
Income from continuing operations	173,632,000	173,294,000	321,782,000	327,430,000
Gain (loss) on real estate dispositions, net	10,759,000	42,155,000	134,156,000	273,236,000
Net income	184,391,000	215,449,000	455,938,000	600,666,000
Total assets	8,735,524,000		8,735,524,000
Operating Segments | Senior housing - operating
Segment Reporting Information [Line Items]
Total revenues	765,167,000	678,089,000	1,502,334,000	1,349,956,000
Property operating expenses	525,662,000	459,111,000	1,037,603,000	921,536,000
Net Operating Income from Continuing Operations	239,505,000	218,978,000	464,731,000	428,420,000
Interest expense	16,971,000	15,403,000	33,906,000	31,219,000
Loss (gain) on derivatives and financial instruments, net	0	0	0	0
Depreciation and amortization	134,779,000	117,198,000	260,548,000	236,935,000
General and administrative	0	0	0	0
Loss (gain) on extinguishment of debt, net	299,000	2,524,000	110,000	3,414,000
Impairment of assets	2,212,000	8,785,000	4,513,000	14,191,000
Other expenses	6,167,000	1,165,000	5,979,000	2,943,000
Income (loss) from continuing operations before income taxes and income from unconsolidated entities	79,077,000	73,903,000	159,675,000	139,718,000
Income tax expense	(2,617,000)	10,247,000	(2,455,000)	9,160,000
(Loss) income from unconsolidated entities	(5,204,000)	(8,449,000)	(14,684,000)	(37,640,000)
Income from continuing operations	71,256,000	75,701,000	142,536,000	111,238,000
Gain (loss) on real estate dispositions, net	(1,000)	0	4,000	13,011,000
Net income	71,255,000	75,701,000	142,540,000	124,249,000
Total assets	13,785,165,000		13,785,165,000
Operating Segments | Outpatient Medical
Segment Reporting Information [Line Items]
Total revenues	135,827,000	138,927,000	272,498,000	279,391,000
Property operating expenses	42,953,000	42,744,000	87,456,000	90,488,000
Net Operating Income from Continuing Operations	92,874,000	96,183,000	185,042,000	188,903,000
Interest expense	1,656,000	2,122,000	3,332,000	4,413,000
Loss (gain) on derivatives and financial instruments, net	0	0	0	0
Depreciation and amortization	46,187,000	47,478,000	92,587,000	96,408,000
General and administrative	0	0	0	0
Loss (gain) on extinguishment of debt, net	0	2,991,000	11,928,000	4,373,000
Impairment of assets	0	0	0	5,625,000
Other expenses	2,095,000	1,310,000	2,693,000	1,671,000
Income (loss) from continuing operations before income taxes and income from unconsolidated entities	42,936,000	42,282,000	74,502,000	76,413,000
Income tax expense	(378,000)	(351,000)	(806,000)	(686,000)
(Loss) income from unconsolidated entities	1,391,000	604,000	2,621,000	1,051,000
Income from continuing operations	43,949,000	42,535,000	76,317,000	76,778,000
Gain (loss) on real estate dispositions, net	(3,000)	0	214,779,000	0
Net income	43,946,000	42,535,000	291,096,000	76,778,000
Total assets	4,886,011,000		4,886,011,000
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total revenues	498,000	239,000	866,000	472,000
Property operating expenses	0	0	0	0
Net Operating Income from Continuing Operations	498,000	239,000	866,000	472,000
Interest expense	98,989,000	96,191,000	199,711,000	191,170,000
Loss (gain) on derivatives and financial instruments, net	0	0	0	0
Depreciation and amortization	0	0	0	0
General and administrative	32,831,000	32,632,000	66,536,000	63,733,000
Loss (gain) on extinguishment of debt, net	0	0	0	0
Impairment of assets	0	0	0	0
Other expenses	839,000	1,683,000	3,021,000	6,210,000
Income (loss) from continuing operations before income taxes and income from unconsolidated entities	(132,161,000)	(130,267,000)	(268,402,000)	(260,641,000)
Income tax expense	(158,000)	23,000	(344,000)	0
(Loss) income from unconsolidated entities	0	0	0	0
Income from continuing operations	(132,319,000)	(130,244,000)	(268,746,000)	(260,641,000)
Gain (loss) on real estate dispositions, net	0	0	0	0
Net income	(132,319,000)	(130,244,000)	(268,746,000)	(260,641,000)
Total assets	212,282,000		212,282,000
Rental income
Segment Reporting Information [Line Items]
Total revenues	333,601,000	355,599,000	676,970,000	722,741,000
Rental income | Operating Segments | Triple Net
Segment Reporting Information [Line Items]
Total revenues	197,961,000	217,889,000	404,792,000	445,180,000
Rental income | Operating Segments | Senior housing - operating
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Rental income | Operating Segments | Outpatient Medical
Segment Reporting Information [Line Items]
Total revenues	135,640,000	137,710,000	272,178,000	277,561,000
Rental income | Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Resident fees and services
Segment Reporting Information [Line Items]
Total revenues	763,345,000	677,040,000	1,499,279,000	1,347,377,000
Resident fees and services | Operating Segments | Triple Net
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Resident fees and services | Operating Segments | Senior housing - operating
Segment Reporting Information [Line Items]
Total revenues	763,345,000	677,040,000	1,499,279,000	1,347,377,000
Resident fees and services | Operating Segments | Outpatient Medical
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Resident fees and services | Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Interest income
Segment Reporting Information [Line Items]
Total revenues	13,462,000	20,901,000	28,110,000	41,649,000
Interest income | Operating Segments | Triple Net
Segment Reporting Information [Line Items]
Total revenues	13,247,000	20,901,000	27,798,000	41,580,000
Interest income | Operating Segments | Senior housing - operating
Segment Reporting Information [Line Items]
Total revenues	172,000	0	257,000	69,000
Interest income | Operating Segments | Outpatient Medical
Segment Reporting Information [Line Items]
Total revenues	43,000	0	55,000	0
Interest income | Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Other income
Segment Reporting Information [Line Items]
Total revenues	15,504,000	5,062,000	18,518,000	9,133,000
Other income | Operating Segments | Triple Net
Segment Reporting Information [Line Items]
Total revenues	13,212,000	2,557,000	14,589,000	4,321,000
Other income | Operating Segments | Senior housing - operating
Segment Reporting Information [Line Items]
Total revenues	1,650,000	1,049,000	2,798,000	2,510,000
Other income | Operating Segments | Outpatient Medical
Segment Reporting Information [Line Items]
Total revenues	144,000	1,217,000	265,000	1,830,000
Other income | Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total revenues	$ 498,000	$ 239,000	$ 866,000	$ 472,000