Indebtedness Secured Debt (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) loans	Dec. 31, 2017 USD ($) loans	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
gross basis of secured properties	$ 220,100
Letters of Credit Outstanding, Amount	2,200
Fair value	$ 2,662,401	$ 2,518,801
Debt Instrument, Interest Rate, Effective Percentage	4.13%	3.39%
Repayments of Secured Debt	$ 232,234	$ 72,648	$ 354,832
Repaid Debt, Weighted Average Interest Rate	7.63%
Fixed Rate Secured Debt [Member]
Debt Instrument [Line Items]
Fair value	$ 80,238	$ 325,753
number of secured loans repaid | loans	3	8
Repayments of Secured Debt	$ 227,100	$ 66,500
Repaid Debt, Weighted Average Interest Rate	7.62%	5.85%
Minimum [Member] | Fixed Rate Secured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	4.00%