Segment Information (Income Statement by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues
Rental income									$ 1,476,176	$ 1,346,046	$ 1,138,457
Resident fees and services									1,847,306	1,811,255	1,552,951
Office building and other services revenue									21,070	41,492	29,364
Income from loans and investments									98,094	86,553	51,778
Interest and other income									876	1,052	4,263
Total revenues	$ 875,713	$ 867,116	$ 848,404	$ 852,289	$ 841,274	$ 827,606	$ 811,920	$ 805,598	3,443,522	3,286,398	2,776,813
Less:
Interest and other income									876	1,052	4,263
Property-level operating expenses									1,434,762	1,383,640	1,195,388
Office building services costs									7,311	26,565	17,092
Segment NOI									2,000,573	1,875,141	1,560,070
Income (loss) from unconsolidated entities									4,358	(1,420)	(139)
Segment profit									2,004,931	1,873,721	1,559,931
Interest and other income									876	1,052	4,263
Interest expense									(419,740)	(367,114)	(292,065)
Depreciation and amortization									(898,924)	(894,057)	(725,216)
General, administrative and professional fees									(126,875)	(128,035)	(121,738)
Loss on extinguishment of debt, net									(2,779)	(14,411)	(5,564)
Merger-related expenses and deal costs									(24,635)	(102,944)	(43,304)
Other									(9,988)	(17,957)	(25,743)
Income tax benefit									31,343	39,284	8,732
Income from continuing operations	$ 207,804	$ 149,570	$ 143,310	$ 149,469	$ 127,059	$ 45,235	$ 131,578	$ 102,868	554,209	389,539	359,296
Operating Segments | Triple-net leased properties
Revenues
Rental income									845,834	779,801	674,547
Resident fees and services									0	0	0
Office building and other services revenue									4,921	4,433	4,565
Income from loans and investments									0	0	0
Interest and other income									0	0	0
Total revenues									850,755	784,234	679,112
Less:
Interest and other income									0	0	0
Property-level operating expenses									0	0	0
Office building services costs									0	0	0
Segment NOI									850,755	784,234	679,112
Income (loss) from unconsolidated entities									2,363	(813)	859
Segment profit									853,118	783,421	679,971
Interest and other income									0	0	0
Operating Segments | Senior living operations
Revenues
Rental income									0	0	0
Resident fees and services									1,847,306	1,811,255	1,552,951
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									1,847,306	1,811,255	1,552,951
Less:
Interest and other income									0	0	0
Property-level operating expenses									1,242,978	1,209,415	1,036,556
Office building services costs									0	0	0
Segment NOI									604,328	601,840	516,395
Income (loss) from unconsolidated entities									1,265	(526)	(658)
Segment profit									605,593	601,314	515,737
Interest and other income									0	0	0
Operating Segments | Office operations
Revenues
Rental income									630,342	566,245	463,910
Resident fees and services									0	0	0
Office building and other services revenue									13,029	34,436	22,529
Income from loans and investments									0	0	0
Interest and other income									0	0	0
Total revenues									643,371	600,681	486,439
Less:
Interest and other income									0	0	0
Property-level operating expenses									191,784	174,225	158,832
Office building services costs									7,311	26,565	17,092
Segment NOI									444,276	399,891	310,515
Income (loss) from unconsolidated entities									590	369	398
Segment profit									444,866	400,260	310,913
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									3,120	2,623	2,270
Income from loans and investments									98,094	86,553	51,778
Interest and other income									876	1,052	4,263
Total revenues									102,090	90,228	58,311
Less:
Interest and other income									876	1,052	4,263
Property-level operating expenses									0	0	0
Office building services costs									0	0	0
Segment NOI									101,214	89,176	54,048
Income (loss) from unconsolidated entities									140	(450)	(738)
Segment profit									101,354	88,726	53,310
Interest and other income									$ 876	$ 1,052	$ 4,263