Segment Reporting (Properties and Other Investments by Geographic Location) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Amount(1)	$ 1,262,822	$ 1,266,133	$ 1,320,106	$ 1,272,245	$ 1,241,243	$ 1,236,379	$ 1,125,912	$ 1,096,965	$ 5,121,306	$ 4,700,499	$ 4,316,641
Assets:
Amount	33,380,751				30,342,072				33,380,751	30,342,072
United States
Revenues:
Amount(1)									4,205,492	3,777,960	3,464,527
Assets:
Amount	27,513,911				24,884,292				27,513,911	24,884,292
United Kingdom
Revenues:
Amount(1)									452,698	452,956	407,351
Assets:
Amount	3,405,388				3,078,994				3,405,388	3,078,994
Canada
Revenues:
Amount(1)									463,116	469,583	$ 444,763
Assets:
Amount	$ 2,461,452				$ 2,378,786				$ 2,461,452	$ 2,378,786
Revenue
Assets:
Percentage									100.00%	100.00%	100.00%
Revenue | United States
Assets:
Percentage									82.10%	80.40%	80.30%
Revenue | United States | Resident fees and services
Assets:
Percentage									77.00%
Revenue | United Kingdom
Assets:
Percentage									8.80%	9.60%	9.40%
Revenue | United Kingdom | Resident fees and services
Assets:
Percentage									10.00%
Revenue | Canada
Assets:
Percentage									9.10%	10.00%	10.30%
Revenue | Canada | Resident fees and services
Assets:
Percentage									13.00%
Assets
Assets:
Percentage									100.00%	100.00%
Assets | United States
Assets:
Percentage									82.40%	82.00%
Assets | United Kingdom
Assets:
Percentage									10.20%	10.10%
Assets | Canada
Assets:
Percentage									7.40%	7.90%