Segment Reporting (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Total revenues	$ 1,236,379,000	$ 1,091,483,000	$ 3,459,256,000	$ 3,212,383,000
Property operating expenses	657,157,000	523,997,000	1,782,373,000	1,536,021,000
Net Operating Income from Continuing Operations	579,222,000	567,486,000	1,676,883,000	1,676,362,000
Interest expense	138,032,000	122,578,000	382,223,000	357,405,000
Loss (gain) on derivatives and financial instruments, net	8,991,000	324,000	(5,642,000)	2,284,000
Depreciation and amortization	243,149,000	230,138,000	707,625,000	683,262,000
General and administrative	28,746,000	29,913,000	95,282,000	93,643,000
Loss (gain) on extinguishment of debt, net	4,038,000	0	16,044,000	36,870,000
Impairment of assets	6,740,000	0	39,557,000	24,662,000
Other expenses	88,626,000	99,595,000	102,396,000	117,608,000
Income (loss) from continuing operations before income taxes and income from unconsolidated entities	60,900,000	84,938,000	339,398,000	360,628,000
Income tax expense	(1,741,000)	(669,000)	(7,170,000)	5,535,000
(Loss) income from unconsolidated entities	344,000	3,408,000	(836,000)	(23,676,000)
Income from continuing operations	59,503,000	87,677,000	331,392,000	342,487,000
Gain (loss) on real estate dispositions, net	24,723,000	1,622,000	373,662,000	287,869,000
Net income	84,226,000	$ 89,299,000	705,054,000	$ 630,356,000
Total assets	$ 30,249,119,000		$ 30,249,119,000		$ 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	$ 1,007,203,000	$ 871,431,000	$ 2,766,726,000	$ 2,582,042,000
Total assets	$ 24,616,066,000		$ 24,616,066,000		$ 22,274,443,000
Percent Of Assets	0.814		0.814		0.797
Percent Of Revenues	0.815	0.799	0.800	0.804
United Kingdom
Segment Reporting Information [Line Items]
Total revenues	$ 111,503,000	$ 105,028,000	$ 340,059,000	$ 298,618,000
Total assets	$ 3,150,305,000		$ 3,150,305,000		$ 3,239,039,000
Percent Of Assets	0.104		0.104		0.116
Percent Of Revenues	0.090	0.096	0.098	0.093
Canada
Segment Reporting Information [Line Items]
Total revenues	$ 117,673,000	$ 115,024,000	$ 352,471,000	$ 331,723,000
Total assets	$ 2,482,748,000		$ 2,482,748,000		$ 2,430,963,000
Percent Of Assets	0.082		0.082		0.087
Percent Of Revenues	0.095	0.105	0.102	0.103
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	219,110,000	$ 244,916,000	$ 666,289,000	$ 735,998,000
Property operating expenses	426,000	0	583,000	0
Net Operating Income from Continuing Operations	218,684,000	244,916,000	665,706,000	735,998,000
Interest expense	3,500,000	3,622,000	10,742,000	11,647,000
Loss (gain) on derivatives and financial instruments, net	8,991,000	324,000	(5,642,000)	2,284,000
Depreciation and amortization	60,383,000	62,891,000	171,724,000	182,672,000
General and administrative	0	0	0	0
Loss (gain) on extinguishment of debt, net	0		(32,000)	29,083,000
Impairment of assets	6,178,000		34,482,000	4,846,000
Other expenses	87,076,000	89,236,000	89,153,000	96,425,000
Income (loss) from continuing operations before income taxes and income from unconsolidated entities	52,556,000	88,843,000	365,279,000	409,041,000
Income tax expense	1,116,000	(816,000)	(708,000)	(2,070,000)
(Loss) income from unconsolidated entities	5,377,000	5,478,000	16,260,000	14,983,000
Income from continuing operations	59,049,000	93,505,000	380,831,000	421,954,000
Gain (loss) on real estate dispositions, net	24,782,000	(185,000)	158,938,000	273,051,000
Net income	83,831,000	93,320,000	539,769,000	695,005,000
Total assets	10,163,867,000		10,163,867,000
Operating Segments | Senior housing - operating
Segment Reporting Information [Line Items]
Total revenues	876,505,000	703,877,000	2,378,839,000	2,053,831,000
Property operating expenses	610,659,000	478,777,000	1,648,262,000	1,400,313,000
Net Operating Income from Continuing Operations	265,846,000	225,100,000	730,577,000	653,518,000
Interest expense	17,319,000	16,369,000	51,225,000	47,587,000
Loss (gain) on derivatives and financial instruments, net	0	0	0	0
Depreciation and amortization	136,532,000	119,089,000	397,080,000	356,023,000
General and administrative	0	0	0	0
Loss (gain) on extinguishment of debt, net	0		110,000	3,414,000
Impairment of assets	562,000		5,075,000	14,191,000
Other expenses	(811,000)	5,157,000	5,168,000	8,100,000
Income (loss) from continuing operations before income taxes and income from unconsolidated entities	112,244,000	84,485,000	271,919,000	224,203,000
Income tax expense	211,000	(1,519,000)	(2,244,000)	9,133,000
(Loss) income from unconsolidated entities	(6,705,000)	(2,886,000)	(21,389,000)	(40,527,000)
Income from continuing operations	105,750,000	80,080,000	248,286,000	192,809,000
Gain (loss) on real estate dispositions, net	(1,000)	(197,000)	3,000	12,814,000
Net income	105,749,000	79,883,000	248,289,000	205,623,000
Total assets	14,989,442,000		14,989,442,000
Operating Segments | Outpatient Medical
Segment Reporting Information [Line Items]
Total revenues	140,069,000	141,992,000	412,567,000	421,383,000
Property operating expenses	46,072,000	45,220,000	133,528,000	135,708,000
Net Operating Income from Continuing Operations	93,997,000	96,772,000	279,039,000	285,675,000
Interest expense	1,643,000	2,929,000	4,975,000	7,342,000
Loss (gain) on derivatives and financial instruments, net	0	0	0	0
Depreciation and amortization	46,234,000	48,158,000	138,821,000	144,567,000
General and administrative	0	0	0	0
Loss (gain) on extinguishment of debt, net	0		11,928,000	4,373,000
Impairment of assets	0		0	5,625,000
Other expenses	1,055,000	530,000	3,748,000	2,201,000
Income (loss) from continuing operations before income taxes and income from unconsolidated entities	45,065,000	45,155,000	119,567,000	121,567,000
Income tax expense	239,000	(366,000)	(567,000)	(655,000)
(Loss) income from unconsolidated entities	1,672,000	816,000	4,293,000	1,868,000
Income from continuing operations	46,976,000	45,605,000	123,293,000	122,780,000
Gain (loss) on real estate dispositions, net	(58,000)	2,004,000	214,721,000	2,004,000
Net income	46,918,000	47,609,000	338,014,000	124,784,000
Total assets	4,953,277,000		4,953,277,000
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total revenues	695,000	698,000	1,561,000	1,171,000
Property operating expenses	0	0	0	0
Net Operating Income from Continuing Operations	695,000	698,000	1,561,000	1,171,000
Interest expense	115,570,000	99,658,000	315,281,000	290,829,000
Loss (gain) on derivatives and financial instruments, net	0	0	0	0
Depreciation and amortization	0	0	0	0
General and administrative	28,746,000	29,913,000	95,282,000	93,643,000
Loss (gain) on extinguishment of debt, net	4,038,000		4,038,000	0
Impairment of assets	0		0	0
Other expenses	1,306,000	4,672,000	4,327,000	10,882,000
Income (loss) from continuing operations before income taxes and income from unconsolidated entities	(148,965,000)	(133,545,000)	(417,367,000)	(394,183,000)
Income tax expense	(3,307,000)	2,032,000	(3,651,000)	(873,000)
(Loss) income from unconsolidated entities	0	0	0	0
Income from continuing operations	(152,272,000)	(131,513,000)	(421,018,000)	(395,056,000)
Gain (loss) on real estate dispositions, net	0	0	0	0
Net income	(152,272,000)	(131,513,000)	(421,018,000)	(395,056,000)
Total assets	142,533,000		142,533,000
Rental income
Segment Reporting Information [Line Items]
Total revenues	342,887,000	362,880,000	1,019,857,000	1,085,621,000
Rental income | Operating Segments | Triple Net
Segment Reporting Information [Line Items]
Total revenues	203,039,000	221,555,000	607,831,000	666,735,000
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	139,848,000	141,325,000	412,026,000	418,886,000
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	875,171,000	702,380,000	2,374,450,000	2,049,757,000
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	875,171,000	702,380,000	2,374,450,000	2,049,757,000
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	14,622,000	20,187,000	42,732,000	61,836,000
Interest income | Operating Segments | Triple Net
Segment Reporting Information [Line Items]
Total revenues	14,378,000	20,187,000	42,176,000	61,767,000
Interest income | Operating Segments | Senior housing - operating
Segment Reporting Information [Line Items]
Total revenues	159,000	0	416,000	69,000
Interest income | Operating Segments | Outpatient Medical
Segment Reporting Information [Line Items]
Total revenues	85,000	0	140,000	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	3,699,000	6,036,000	22,217,000	15,169,000
Other income | Operating Segments | Triple Net
Segment Reporting Information [Line Items]
Total revenues	1,693,000	3,174,000	16,282,000	7,496,000
Other income | Operating Segments | Senior housing - operating
Segment Reporting Information [Line Items]
Total revenues	1,175,000	1,497,000	3,973,000	4,005,000
Other income | Operating Segments | Outpatient Medical
Segment Reporting Information [Line Items]
Total revenues	136,000	667,000	401,000	2,497,000
Other income | Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total revenues	$ 695,000	$ 698,000	$ 1,561,000	$ 1,171,000