Segment Reporting (Properties and Other Investments by Geographic Location) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues:
Amount	$ 1,122,016	$ 1,036,874	$ 1,188,475	$ 1,258,602	$ 1,262,822	$ 1,266,133	$ 1,320,106	$ 1,272,245	$ 4,605,967	$ 5,121,306	$ 4,700,499
Assets:
Amount	32,483,642				33,380,751				32,483,642	33,380,751
United States
Revenues:
Amount									3,720,155	4,205,492	3,777,960
Assets:
Amount	26,658,659				27,513,911				26,658,659	27,513,911
United Kingdom
Revenues:
Amount									451,399	452,698	452,956
Assets:
Amount	3,352,549				3,405,388				3,352,549	3,405,388
Canada
Revenues:
Amount									434,413	463,116	$ 469,583
Assets:
Amount	$ 2,472,434				$ 2,461,452				$ 2,472,434	$ 2,461,452
Revenue
Assets:
Percentage									100.00%	100.00%	100.00%
Revenue | United States
Assets:
Percentage									80.80%	82.10%	80.40%
Revenue | United States | Resident fees and services
Assets:
Percentage									76.00%
Revenue | United Kingdom
Assets:
Percentage									9.80%	8.80%	9.60%
Revenue | United Kingdom | Resident fees and services
Assets:
Percentage									10.00%
Revenue | Canada
Assets:
Percentage									9.40%	9.10%	10.00%
Revenue | Canada | Resident fees and services
Assets:
Percentage									14.00%
Assets
Assets:
Percentage									100.00%	100.00%
Assets | United States
Assets:
Percentage									82.10%	82.40%
Assets | United Kingdom
Assets:
Percentage									10.30%	10.20%
Assets | Canada
Assets:
Percentage									7.60%	7.40%