Segment Information (Income Statement by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Rental income									$ 1,513,807	$ 1,593,598	$ 1,476,176
Income from loans and investments									124,218	117,608	98,094
Interest and other income									24,892	6,034	876
Total revenues	$ 923,263	$ 936,538	$ 942,304	$ 943,705	$ 895,288	$ 899,928	$ 895,490	$ 883,443	3,745,810	3,574,149	3,443,522
Less:
Interest and other income									24,892	6,034	876
Property-level operating expenses									1,689,880	1,483,072	1,434,762
Office building services costs									1,418	3,391	7,311
Segment NOI									2,029,620	2,081,652	2,000,573
(Loss) income from unconsolidated entities									(55,034)	(561)	4,358
Segment profit									1,974,586	2,081,091	2,004,931
Interest and other income									24,892	6,034	876
Interest expense									(442,497)	(448,196)	(419,740)
Depreciation and amortization									(919,639)	(887,948)	(898,924)
General, administrative and professional fees									(151,982)	(135,490)	(126,875)
Loss on extinguishment of debt, net									(58,254)	(754)	(2,779)
Merger-related expenses and deal costs									(30,547)	(10,535)	(24,635)
Other									(66,768)	(20,052)	(9,988)
Gain on real estate dispositions									46,247	717,273	98,203
Income tax benefit									39,953	59,799	31,343
Income from continuing operations									415,991	1,361,222	652,412
Operating Segments | Triple Net Leased Properties
Revenues
Rental income									737,796	840,131	845,834
Income from loans and investments									0	0	0
Interest and other income									0	0	0
Total revenues									740,318	844,711	850,755
Less:
Interest and other income									0	0	0
Property-level operating expenses									0	0	0
Office building services costs									0	0	0
Segment NOI									740,318	844,711	850,755
(Loss) income from unconsolidated entities									(47,901)	845	2,363
Segment profit									692,417	845,556	853,118
Interest and other income									0	0	0
Operating Segments | Senior Living Operations
Revenues
Rental income									0	0	0
Income from loans and investments									0	0	0
Interest and other income									0	0	0
Total revenues									2,069,477	1,843,232	1,847,306
Less:
Interest and other income									0	0	0
Property-level operating expenses									1,446,201	1,250,065	1,242,978
Office building services costs									0	0	0
Segment NOI									623,276	593,167	604,328
(Loss) income from unconsolidated entities									(4,465)	(61)	1,265
Segment profit									618,811	593,106	605,593
Interest and other income									0	0	0
Operating Segments | Office Operations
Revenues
Rental income									776,011	753,467	630,342
Income from loans and investments									0	0	0
Interest and other income									0	0	0
Total revenues									783,603	760,964	643,371
Less:
Interest and other income									0	0	0
Property-level operating expenses									243,679	233,007	191,784
Office building services costs									1,418	3,391	7,311
Segment NOI									538,506	524,566	444,276
(Loss) income from unconsolidated entities									477	503	590
Segment profit									538,983	525,069	444,866
Interest and other income									0	0	0
All Other
Revenues
Rental income									0	0	0
Income from loans and investments									124,218	117,608	98,094
Interest and other income									24,892	6,034	876
Total revenues									152,412	125,242	102,090
Less:
Interest and other income									24,892	6,034	876
Property-level operating expenses									0	0	0
Office building services costs									0	0	0
Segment NOI									127,520	119,208	101,214
(Loss) income from unconsolidated entities									(3,145)	(1,848)	140
Segment profit									124,375	117,360	101,354
Interest and other income									24,892	6,034	876
Resident Fees and Services
Revenues
Revenue from contracts with customers									2,069,477	1,843,232	1,847,306
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									2,069,477	1,843,232	1,847,306
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									13,416	13,677	21,070
Office Building and Other Services Revenue | Operating Segments | Triple Net Leased Properties
Revenues
Revenue from contracts with customers									2,522	4,580	4,921
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									7,592	7,497	13,029
Office Building and Other Services Revenue | All Other
Revenues
Revenue from contracts with customers									$ 3,302	$ 1,600	$ 3,120