Long-Term Debt - Schedule of Interest Expense Related To Debt (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Disclosure [Abstract]
Interest expense	$ 11,550,000	$ 9,985,000	$ 32,601,000	$ 28,210,000
Amortization of debt issuance costs related to long-term debt, including our credit facility	541,000	830,000	2,106,000	2,503,000
Commitment fees	681,000	674,000	2,005,000	2,236,000
Interest costs capitalized	$ 0	$ 0	$ 0	$ 0
Unused capacity, commitment fee percentage			0.30%