Indebtedness (Details)		1 Months Ended																	3 Months Ended			5 Months Ended	7 Months Ended	12 Months Ended
	Sep. 28, 2015	Oct. 31, 2016 USD ($) property	Sep. 30, 2016 USD ($) property note	Jul. 31, 2016 USD ($) building property	Apr. 30, 2016 USD ($) property	Feb. 29, 2016 USD ($)	Jan. 31, 2016 USD ($) property	Nov. 30, 2015 USD ($)	Oct. 31, 2015 USD ($) living_unit mortgage	Sep. 30, 2015 community	Jun. 30, 2015 USD ($) living_unit	May 31, 2015 USD ($) living_unit community mortgage	Apr. 30, 2015 USD ($)	Feb. 28, 2015 USD ($)	Jan. 31, 2015 USD ($) living_unit building	Dec. 31, 2014 USD ($) community	Dec. 31, 2014 USD ($) lease_agreement	May 31, 2014	Dec. 31, 2016 USD ($) property building note	Dec. 31, 2015 USD ($) property	Mar. 31, 2015 USD ($)	Sep. 30, 2015 community	Dec. 31, 2014 USD ($)	Dec. 31, 2016 USD ($) property building note	Dec. 31, 2015 USD ($) property	Dec. 31, 2014 USD ($) community property	Feb. 24, 2017 USD ($)
Indebtedness
Line of credit facility required principal payment before maturity																								$ 0
Number of public issues of unsecured senior notes | note																			6					6
Senior unsecured notes, net																			$ 1,722,758,000	$ 1,478,536,000				$ 1,722,758,000	$ 1,478,536,000
Unsecured term loan facility																			1,750,000,000	1,500,000,000				1,750,000,000	1,500,000,000
Carrying value of mortgaged properties																			6,402,512,000	6,309,400,000				6,402,512,000	6,309,400,000
Real estate property, gross																			7,730,523,000	7,456,940,000				7,730,523,000	7,456,940,000
Capital leases																			11,500,000					11,500,000
Assumption of mortgage notes payable																								0	181,433,000	$ 15,630,000
Unsecured revolving credit facility																			327,000,000	$ 775,000,000				327,000,000	775,000,000
Loss on early extinguishment of debt																					$ 1,409,000			526,000	1,894,000	12,000
Sale of senior unsecured notes																								0	$ 200,000,000	$ 350,000,000
Unsecured revolving credit facility
Indebtedness
Unsecured revolving credit facility, maximum borrowing capacity																			1,000,000,000					1,000,000,000
Revolving credit facility, available amount																			673,000,000					673,000,000
Option to increase the borrowing capacity under revolving credit facility																			$ 1,500,000,000					$ 1,500,000,000
Interest rate added to the base rate (as a percent)																									1.30%
Revolving credit facility, interest rate payable (as a percent)																			2.00%					2.00%
Weighted average interest rate on debt (as a percent)																1.40%	1.40%		1.80%	1.50%			1.40%	1.80%	1.50%	1.40%
Interest expense and other associated costs incurred																								$ 11,235,000	$ 9,252,000	$ 3,094,000
Debt instrument, facility fee (as a percent)																									0.30%
Unsecured revolving credit facility																			$ 327,000,000					327,000,000
Senior unsecured notes 4.30%
Indebtedness
Senior unsecured notes, net																			$ 250,000,000					$ 250,000,000
Interest rate (as a percent)																			4.30%					4.30%
Repayments of secured debt								$ 250,000,000
Loss on early extinguishment of debt																									$ 175,000
Senior unsecured notes 6.75%
Indebtedness
Senior unsecured notes, net																			$ 200,000,000					$ 200,000,000
Interest rate (as a percent)																			6.75%					6.75%
Unsecured term loan facility																			$ 200,000,000	$ 200,000,000				$ 200,000,000	200,000,000
Unsecured senior notes due 2021
Indebtedness
Senior unsecured notes, net																			$ 300,000,000					$ 300,000,000
Interest rate (as a percent)																			6.75%					6.75%
Unsecured term loan facility																			$ 300,000,000	300,000,000				$ 300,000,000	300,000,000
Senior unsecured notes 5.625% due in 2042
Indebtedness
Senior unsecured notes, net																			$ 350,000,000					$ 350,000,000
Interest rate (as a percent)																			5.625%					5.625%
Unsecured term loan facility																			$ 350,000,000	$ 350,000,000				$ 350,000,000	350,000,000
Senior unsecured notes 6.25 percent due 2046
Indebtedness
Senior unsecured notes, net																			$ 250,000,000					$ 250,000,000
Interest rate (as a percent)						6.25%													6.25%					6.25%
Sale of senior unsecured notes						$ 250,000,000
Mortgages
Indebtedness
Aggregate principal amount of mortgage debt																			$ 1,109,807,000					$ 1,109,807,000
Number of properties mortgaged | property		8		1	1		1												43					43
Number of buildings mortgaged | building																			45					45
Carrying value of mortgaged properties																			$ 1,618,222,000					$ 1,618,222,000
Principal repayment due until maturity											$ 4,351,000
Repayments of secured debt									$ 52,000,000		$ 4,867,000	$ 15,077,000	$ 6,274,000	$ 29,227,000
Weighted average interest rate on debt (as a percent)											5.65%
Weighted average interest rate on debt (as a percent)														6.02%
Weighted average interest rate on prepaid debt (as a percent)				6.25%	4.65%		5.97%		5.64%		5.81%	5.70%	5.81%
Prepayment Of Mortgage				$ 11,871,000	$ 18,000,000		$ 6,115,000
Loss on early extinguishment of debt																									$ 290,000
Number of mortgage notes | mortgage									2			4
Number of properties encumbered under mortgage notes					1				1		1	4
Capital leases
Indebtedness
Interest rate (as a percent)																			7.70%					7.70%
Number of properties mortgaged | property																				2					2
Carrying value of mortgaged properties																			$ 18,968,000	$ 19,400,000				$ 18,968,000	$ 19,400,000
Number of properties recorded under capital lease | property																			2					2
Real estate property, gross																			$ 36,084,000					$ 36,084,000
Capital leases																			11,466,000					11,466,000
Senior unsecured notes due 2019
Indebtedness
Senior unsecured notes, net																			$ 400,000,000					$ 400,000,000
Interest rate (as a percent)																			3.25%					3.25%
Senior unsecured notes due 2024
Indebtedness
Interest rate (as a percent)																			4.75%					4.75%
Senior unsecured notes 4.75%
Indebtedness
Unsecured term loan facility																			$ 250,000,000	$ 250,000,000				$ 250,000,000	$ 250,000,000
Unsecured debt
Indebtedness
Interest rate added to the base rate (as a percent)	1.80%																	1.40%
Weighted average interest rate on debt (as a percent)																				2.00%			1.60%	1.90%	1.60%
Maximum borrowing capacity that may be increased																			$ 400,000,000					$ 400,000,000
Term loan due 2020
Indebtedness
Interest rate (as a percent)																			2.00%					2.00%
Unsecured term loan facility																			$ 350,000,000					$ 350,000,000
Interest expense and other associated costs incurred																								6,721,000	$ 5,686,000	$ 3,263,000
Maximum borrowing capacity that may be increased																			700,000,000					700,000,000
Term loan due 2022
Indebtedness
Unsecured term loan facility																			$ 200,000,000					$ 200,000,000
Weighted average interest rate on debt (as a percent)																			2.60%					2.60%
Weighted average interest rate on debt (as a percent)																								2.30%
Unsecured debt September 2015
Indebtedness
Interest expense and other associated costs incurred																								$ 4,645,000	$ 1,061,000
Prepayment Notification | Mortgages
Indebtedness
Number of properties mortgaged | property			2																1					1
Weighted average interest rate on prepaid debt (as a percent)		6.54%	5.92%																5.86%
Prepayment Of Mortgage		$ 42,542,000	$ 79,957,000																$ 5,428,000					$ 5,428,000
Number of mortgage notes | note			2																1
Senior Living Communities
Indebtedness
Number of properties acquired or agreed to be acquired | property																									39	2
Number of communities | community																						19
Senior Living Communities | Term loan due 2022
Indebtedness
Senior unsecured notes, net																				$ 200,000,000					$ 200,000,000
Senior Living Communities | Agreement to Acquire
Indebtedness
Number of properties acquired or agreed to be acquired | community										1		37
Assumption of mortgage notes payable												$ 151,477,000
Weighted average interest rate on debt (as a percent)												4.57%
Bridge loan commitment received																$ 700,000,000	$ 700,000,000						$ 700,000,000			$ 700,000,000
Number of communities																38	38									38
MOBs
Indebtedness
Number of properties acquired encumbering with assumed mortgage debt | building															2
Number of properties acquired or agreed to be acquired | living_unit															23
MOBs | Secured debt financing due 2026
Indebtedness
Aggregate principal amount of mortgage debt				$ 620,000,000
Number of properties mortgaged | property				1
Number of buildings mortgaged | building				2
Weighted average interest rate on debt (as a percent)				3.53%
MOBs | Mortgages | Significant Unobservable Inputs (Level 3)
Indebtedness
Assumption of mortgage notes payable															$ 29,955,000
Fair value of mortgage notes payable															$ 31,029,000
Weighted average interest rate on prepaid debt (as a percent)																								4.73%
Subsequent Event | Unsecured revolving credit facility
Indebtedness
Revolving credit facility, available amount																											$ 622,000,000
Unsecured revolving credit facility																											$ 378,000,000