Borrowing Arrangements (Credit Facilities and Unsecured Term Loans) (Details)	1 Months Ended		12 Months Ended
	Aug. 31, 2017 property	Apr. 30, 2017 USD ($) period	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Loss on extinguishment of debt, net			$ (754,000)	$ (2,779,000)	$ (14,411,000)
Long-term debt, gross			11,365,633,000	11,190,914,000
Unsecured Debt
Debt Instrument [Line Items]
Loss on extinguishment of debt, net			(500,000)
Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Long-term debt, gross			$ 535,832,000	$ 146,538,000
Unsecured Revolving Credit Facility | Unsecured Term Loan Due 2019 | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate				1.05%
Unsecured Term Loan Due 2020
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			0.975%
Long-term debt, gross			$ 900,000,000	$ 900,000,000
Unsecured Term Loan Due 2019
Debt Instrument [Line Items]
Extinguishment of debt			278,600,000
Long-term debt, gross			0	371,215,000
Unsecured Term Loan Due 2019 | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.05%
Secured Revolving Construction Credit Facility Due 2022
Debt Instrument [Line Items]
Long-term debt, gross			2,868,000	$ 0
Line of credit facility, remaining borrowing capacity			2,900,000
Unsecured Term Loan Due 2018
Debt Instrument [Line Items]
Extinguishment of debt			200,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Additional periods | period		2
Additional period term		6 months
Letters of credit outstanding			14,500,000
Line of credit facility, remaining borrowing capacity			2,400,000,000
Revolving Credit Facility | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		0.875%
Revolving Credit Facility | Accordion Feature
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 3,750,000,000.00
Revolving Credit Facility | Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 3,000,000,000
Line of credit, current borrowing capacity			$ 400,000,000
Previous Revolving Credit Facility | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.00%
Previous Revolving Credit Facility | Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 2,000,000,000
Skilled Nursing Facilities
Debt Instrument [Line Items]
Number of real estate properties sold | property	22