Debt (Narrative) (Details) $ in Thousands, beds_or_units in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 properties	Jun. 30, 2021 beds_or_units	Jun. 30, 2021	Dec. 31, 2020 USD ($)
Convertible Debt	$ 0	$ 0					$ 60,000
Debt Leverage Limit, Coupon Change, Trigger		50.00%
Credit Facility, Current Revolving Borrowing Capacity	550,000	$ 550,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage		0.20%
Debt Instrument, Variable Rate Basis, Minimum						1000.00%	1400.00%
Number of Real Estate Properties | properties				222
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	3,600	$ 3,600
Derivative, Fair Value, Net							$ (7,100)
Repayments of Convertible Debt		67,100
Long-term Debt, Gross	1,442,087	1,442,087
Debt Instrument, Convertible, Liquidation Preference, Value	6,100	6,100
Interest on Convertible Debt, Net of Tax	1,000
Proceeds from Issuance of Senior Long-term Debt		396,784	$ 0
Fair Value, Inputs, Level 2 [Member]
Interest rate swap liability	(3,633)	(3,633)	$ (7,150)
$250M Term Loan [Member]
Term Loans, Outstanding Balance	225,000	225,000
$300M Term Loan [Member]
Term Loans, Outstanding Balance	300,000	$ 300,000
Debt Instrument, Name, Revolving Credit Facility [Member]
Debt Instrument, Basis Spread on Variable Rate		1.20%
Unused balance of the unsecured revolving credit facility	525,000	$ 525,000
Bank Term Loans [Member]
Debt Instrument, Basis Spread on Variable Rate		1.32%
Debt Instrument, Name, Fannie Mae Term Loans [Member]
Number of Real Estate Properties						13
Net Book Value of Real Estate Pledged Toward Mortgage	128,000	$ 128,000
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Face Amount of Mortgages	78,100	$ 78,100
Debt Instrument, Interest Rate, Stated Percentage						3.79%
Debt Instrument, Term		10 years
FNMA Berkadia Note [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Face Amount of Mortgages	17,100	$ 17,100
Debt Instrument, Interest Rate, Stated Percentage						4.60%
Debt Instrument, Name, Convertible Senior Notes [Member]
Debt Instrument, Interest Rate, Stated Percentage						3.25%
$100M Term Loan [Member]
Term Loans, Outstanding Balance	100,000	$ 100,000
Debt Instrument, Basis Spread on Variable Rate		1.85%
Debt Instrument, Variable Rate Basis, Minimum						0.50%
Payment for Debt Extinguishment or Debt Prepayment Cost		$ (500)
Revolving Credit Facility [Member]
Debt Instrument, Maturity, Renewal Fee					0.6	10
Senior Notes Due 2031 | Senior Notes
Debt Instrument, Interest Rate, Stated Percentage						3.00%
Debt Instrument, Face Amount	400,000	400,000
Percentage of Issue Price On Face Value						0.99196%
Proceeds from Issuance of Senior Long-term Debt		392,300
Interest Rate Swap(s), Date Entered, March & June 2019 [Member] | Bank Term Loans [Member]
Derivative, Notional Amount	400,000	400,000
Interest Rate Swaps, Maturity Date, After June 2020 [Member]
Derivative, Notional Amount	$ 400,000	$ 400,000