Borrowing Arrangements (Credit Facilities and Unsecured Term Loans) (Details)	1 Months Ended				12 Months Ended
	Aug. 31, 2018 USD ($)	Jul. 31, 2018 USD ($)	Aug. 31, 2017 USD ($) property	Apr. 30, 2017 USD ($) period	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 31, 2019 USD ($)
Debt Instrument [Line Items]
Loss on extinguishment of debt, net					$ (58,254,000)	$ (754,000)	$ (2,779,000)
Long-term debt, gross					10,829,702,000	11,365,633,000
Line of credit facility, amount outstanding					693,600,000
Senior notes payable and other debt					10,733,699,000	11,276,062,000
Unsecured Debt
Debt Instrument [Line Items]
Loss on extinguishment of debt, net			$ (500,000)
Unsecured Term Loan Due 2023
Debt Instrument [Line Items]
Long-term debt, gross		$ 300,000,000.0			300,000,000	0
Unsecured Term Loan Due 2024
Debt Instrument [Line Items]
Long-term debt, gross		$ 600,000,000.0			600,000,000	0
Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Long-term debt, gross					765,919,000	$ 535,832,000
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]
Long-term debt, gross					0	$ 900,000,000
Unsecured Term Loan Due 2020 | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		0.975%
Unsecured Term Loan Due 2019
Debt Instrument [Line Items]
Extinguishment of debt	$ 278,600,000
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					90,488,000	$ 2,868,000
Line of credit facility, remaining borrowing capacity					309,500,000
Line of credit, current borrowing capacity					400,000,000.0
Line of credit facility, amount outstanding					90,500,000
Unsecured Term Loan Due 2018
Debt Instrument [Line Items]
Extinguishment of debt	$ 200,000,000.0
Unsecured Term Loan Due Facility
Debt Instrument [Line Items]
Long-term debt, gross		$ 900,000,000.0
Unsecured Term Loan Due Facility | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		0.90%
Revolving Credit Facility
Debt Instrument [Line Items]
Additional periods | period				2
Additional period term				6 months
Letters of credit outstanding					23,100,000
Line of credit facility, remaining borrowing capacity					2,200,000,000
Line of credit facility, amount outstanding					$ 765,900,000
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
Revolving Credit Facility | Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity				$ 3,000,000,000.0
Unsecured Term Loan Due Facility | Accordion Feature
Debt Instrument [Line Items]
Senior notes payable and other debt		$ 1,500,000,000
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				$ 2,000,000,000.0
Skilled Nursing Facilities
Debt Instrument [Line Items]
Number of real estate properties sold | property			22
Subsequent Event
Debt Instrument [Line Items]
Commercial paper program capacity								$ 1,000,000,000