Segment Information (Income Statement by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Rental income									$ 1,593,598	$ 1,476,176	$ 1,346,046
Resident fees and services									1,843,232	1,847,306	1,811,255
Office building and other services revenue									13,677	21,070	41,492
Income from loans and investments									117,608	98,094	86,553
Interest and other income									6,034	876	1,052
Total revenues	$ 895,288	$ 899,928	$ 895,490	$ 883,443	$ 875,713	$ 867,116	$ 848,404	$ 852,289	3,574,149	3,443,522	3,286,398
Less:
Interest and other income									6,034	876	1,052
Property-level operating expenses									1,483,072	1,434,762	1,383,640
Office building services costs									3,391	7,311	26,565
Segment NOI									2,081,652	2,000,573	1,875,141
(Loss) income from unconsolidated entities									(561)	4,358	(1,420)
Segment profit									2,081,091	2,004,931	1,873,721
Interest and other income									6,034	876	1,052
Interest expense									(448,196)	(419,740)	(367,114)
Depreciation and amortization									(887,948)	(898,924)	(894,057)
General, administrative and professional fees									(135,490)	(126,875)	(128,035)
Loss on extinguishment of debt, net									(754)	(2,779)	(14,411)
Merger-related expenses and deal costs									(10,535)	(24,635)	(102,944)
Other									(20,052)	(9,988)	(17,957)
Income tax benefit									59,799	31,343	39,284
Income from continuing operations									643,949	554,209	389,539
Operating Segments | Triple-Net Leased Properties
Revenues
Rental income									840,131	845,834	779,801
Resident fees and services									0	0	0
Office building and other services revenue									4,580	4,921	4,433
Income from loans and investments									0	0	0
Interest and other income									0	0	0
Total revenues									844,711	850,755	784,234
Less:
Interest and other income									0	0	0
Property-level operating expenses									0	0	0
Office building services costs									0	0	0
Segment NOI									844,711	850,755	784,234
(Loss) income from unconsolidated entities									845	2,363	(813)
Segment profit									845,556	853,118	783,421
Interest and other income									0	0	0
Operating Segments | Senior Living Operations
Revenues
Rental income									0	0	0
Resident fees and services									1,843,232	1,847,306	1,811,255
Office building and other services revenue									0	0	0
Income from loans and investments									0	0	0
Interest and other income									0	0	0
Total revenues									1,843,232	1,847,306	1,811,255
Less:
Interest and other income									0	0	0
Property-level operating expenses									1,250,065	1,242,978	1,209,415
Office building services costs									0	0	0
Segment NOI									593,167	604,328	601,840
(Loss) income from unconsolidated entities									(61)	1,265	(526)
Segment profit									593,106	605,593	601,314
Interest and other income									0	0	0
Operating Segments | Office Operations
Revenues
Rental income									753,467	630,342	566,245
Resident fees and services									0	0	0
Office building and other services revenue									7,497	13,029	34,436
Income from loans and investments									0	0	0
Interest and other income									0	0	0
Total revenues									760,964	643,371	600,681
Less:
Interest and other income									0	0	0
Property-level operating expenses									233,007	191,784	174,225
Office building services costs									3,391	7,311	26,565
Segment NOI									524,566	444,276	399,891
(Loss) income from unconsolidated entities									503	590	369
Segment profit									525,069	444,866	400,260
Interest and other income									0	0	0
All Other
Revenues
Rental income									0	0	0
Resident fees and services									0	0	0
Office building and other services revenue									1,600	3,120	2,623
Income from loans and investments									117,608	98,094	86,553
Interest and other income									6,034	876	1,052
Total revenues									125,242	102,090	90,228
Less:
Interest and other income									6,034	876	1,052
Property-level operating expenses									0	0	0
Office building services costs									0	0	0
Segment NOI									119,208	101,214	89,176
(Loss) income from unconsolidated entities									(1,848)	140	(450)
Segment profit									117,360	101,354	88,726
Interest and other income									$ 6,034	$ 876	$ 1,052