Segment Reporting (Properties and Other Investments by Geographic Location) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Amount	$ 1,241,243	$ 1,236,379	$ 1,125,912	$ 1,096,965	$ 1,104,257	$ 1,091,483	$ 1,058,602	$ 1,062,298	$ 4,700,499	$ 4,316,641	$ 4,281,160
Percentage									1.000	1.000	1.000
Assets:
Amount	$ 30,342,072				$ 27,944,445				$ 30,342,072	$ 27,944,445
Percentage	1.000				1.000				1.000	1.000
United States
Segment Reporting Information [Line Items]
Amount									$ 3,777,960	$ 3,464,527	$ 3,453,485
Percentage									0.804	0.803	0.806
Assets:
Amount	$ 24,884,292				$ 22,274,443				$ 24,884,292	$ 22,274,443
Percentage	0.820				0.797				0.820	0.797
United Kingdom
Segment Reporting Information [Line Items]
Amount									$ 452,956	$ 407,351	$ 388,383
Percentage									0.096	0.094	0.091
Assets:
Amount	$ 3,078,994				$ 3,239,039				$ 3,078,994	$ 3,239,039
Percentage	0.101				0.116				0.101	0.116
Canada
Segment Reporting Information [Line Items]
Amount									$ 469,583	$ 444,763	$ 439,292
Percentage									0.100	0.103	0.103
Assets:
Amount	$ 2,378,786				$ 2,430,963				$ 2,378,786	$ 2,430,963
Percentage	0.079				0.087				0.079	0.087