Debt Obligations - Consolidated Debt Obligations Outstanding (Parenthetical) (Details) - Unsecured Debt [Member] - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 1,721,110	$ 1,628,610
$600 million Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Unsecured credit facility, maximum borrowing capacity		600,000
Long-term Debt, Gross		$ 92,500	0
Debt Instrument Maturity Date		Jul. 15, 2022
Debt Instrument, Credit Spread		LIBOR + 1.10%
Seven Year Term Loan - Swapped to fixed [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 250,000	$ 250,000
Debt Instrument Maturity Date		Oct. 08, 2022
Decrease in interest rate		(0.55%)
Effective interest rate	[1]	2.87%
Seven Year Term Loan - Swapped to fixed [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Credit Spread		LIBOR plus 1.80%
Seven Year Term Loan - Swapped to fixed [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Credit Spread		LIBOR plus 1.25%

[1] On December 13, 2018, the Company amended and restated its $250.0 million seven-year term loan maturing October 8, 2022. In connection with the terms of the amendment, the credit spread on the term loan decreased from LIBOR plus 1.80% to LIBOR plus 1.25%, reducing the Company’s effective interest rate by 0.55%. Through a series of interest rate swaps, the $250.0 million outstanding balance of the term loan has a fixed interest rate of 2.87%.