Notes Payable (Unsecured Term Loan) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Unsecured term loan	$ 1,096,602	$ 1,098,764
Term Loan
Debt Instrument [Line Items]
Unsecured term loan	$ 250,000	$ 250,000
Term Loan | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	1.20%
Term Loan | LIBOR | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	1.20%
Term Loan | LIBOR | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	1.70%
Term Loan | Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	0.50%
Term Loan | One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	1.00%
Term Loan | Base Rate | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	0.00%
Term Loan | Base Rate | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	0.75%