Segment Information (Income Statement by Segment) (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
Rental income									$ 1,494,892	$ 1,609,876	$ 1,513,807
Interest and other income									7,609	10,984	24,892
Total revenues	$ 921,165	$ 918,940	$ 943,198	$ 1,012,054	$ 996,004	$ 983,155	$ 950,717	$ 942,874	3,795,357	3,872,750	3,745,810
Less:
Interest and other income									7,609	10,984	24,892
Property-level operating expenses									1,937,443	1,808,208	1,689,880
Office building services costs									2,315	2,319	1,418
Segment NOI									1,847,990	2,051,239	2,029,620
Interest expense									(469,541)	(451,662)	(442,497)
Depreciation and amortization									(1,109,763)	(1,045,620)	(919,639)
General, administrative and professional fees									(130,158)	(158,726)	(145,978)
Loss on extinguishment of debt, net									(10,791)	(41,900)	(58,254)
Merger-related expenses and deal costs									(29,812)	(15,235)	(30,547)
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease)									(24,238)
Other									707	(10,339)	72,772
Income (loss) from unconsolidated entities									1,844	(2,454)	(55,034)
Gain on real estate dispositions									262,218	26,022	46,247
Income tax benefit									96,534	56,310	39,953
Income from continuing operations									441,185	439,297	415,991
Discontinued operations									0	0	(10)
Net income	111,953	13,737	(159,235)	474,730	12,893	86,918	211,898	127,588	441,185	439,297	415,981
Comprehensive income attributable to noncontrolling interests	1,502	986	(2,065)	1,613	1,450	1,659	1,369	1,803	2,036	6,281	6,514
Net income	$ 110,451	$ 12,751	$ (157,170)	$ 473,117	$ 11,443	$ 85,259	$ 210,529	$ 125,785	439,149	433,016	409,467
Interest and Dividend Income, Operating									80,505	89,201	124,218
Other									707	(10,339)	72,772
Interest and Dividend Income, Operating									80,505	89,201	124,218
Operating Segments | Triple-net leased properties
Revenues
Rental income									695,265	780,898	737,796
Interest and other income									0	0	0
Total revenues									695,265	780,898	740,318
Less:
Interest and other income									0	0	0
Property-level operating expenses									22,160	26,561	0
Office building services costs									0	0	0
Segment NOI									673,105	754,337	740,318
Interest and Dividend Income, Operating									0	0	0
Interest and Dividend Income, Operating									0	0	0
Operating Segments | Senior living operations
Revenues
Rental income									0	0	0
Interest and other income									0	0	0
Total revenues									2,197,160	2,151,533	2,069,477
Less:
Interest and other income									0	0	0
Property-level operating expenses									1,658,671	1,521,398	1,446,201
Office building services costs									0	0	0
Segment NOI									538,489	630,135	623,276
Interest and Dividend Income, Operating									0	0	0
Interest and Dividend Income, Operating									0	0	0
Operating Segments | Office operations
Revenues
Rental income									799,627	828,978	776,011
Interest and other income									0	0	0
Total revenues									808,302	836,725	783,603
Less:
Interest and other income									0	0	0
Property-level operating expenses									256,612	260,249	243,679
Office building services costs									2,315	2,319	1,418
Segment NOI									549,375	574,157	538,506
Interest and Dividend Income, Operating									0	0	0
Interest and Dividend Income, Operating									0	0	0
All Other
Revenues
Rental income									0	0	0
Interest and other income									7,609	10,984	24,892
Total revenues									94,630	103,594	152,412
Less:
Interest and other income									7,609	10,984	24,892
Property-level operating expenses									0	0	0
Office building services costs									0	0	0
Segment NOI									87,021	92,610	127,520
Interest and Dividend Income, Operating									80,505	89,201	124,218
Interest and Dividend Income, Operating									80,505	89,201	124,218
Resident Fees and Services
Revenues
Revenue from contracts with customers									2,197,160	2,151,533	2,069,477
Resident Fees and Services | Operating Segments | Triple-net leased properties
Revenues
Revenue from contracts with customers									0	0	0
Resident Fees and Services | Operating Segments | Senior living operations
Revenues
Revenue from contracts with customers									2,197,160	2,151,533	2,069,477
Resident Fees and Services | Operating Segments | Office operations
Revenues
Revenue from contracts with customers									0	0	0
Resident Fees and Services | All Other
Revenues
Revenue from contracts with customers									0	0	0
Office Building and Other Services Revenue
Revenues
Revenue from contracts with customers									15,191	11,156	13,416
Office Building and Other Services Revenue | Operating Segments | Triple-net leased properties
Revenues
Revenue from contracts with customers									0	0	2,522
Office Building and Other Services Revenue | Operating Segments | Senior living operations
Revenues
Revenue from contracts with customers									0	0	0
Office Building and Other Services Revenue | Operating Segments | Office operations
Revenues
Revenue from contracts with customers									8,675	7,747	7,592
Office Building and Other Services Revenue | All Other
Revenues
Revenue from contracts with customers									$ 6,516	$ 3,409	$ 3,302