Debt - Credit Agreement (Details Textual) - USD ($)		6 Months Ended
	Mar. 05, 2018	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Carrying value of debt		$ 361,404,000		$ 331,736,000
Principal repayments		121,875,000	$ 7,500,000
Net increase of available cash	$ 30,000,000
Payments of deferred financing costs		1,490,000
Financing costs		2,800,000
Loss on extinguishment of debt		(810,000)
2015 Credit Agreement
Debt Instrument [Line Items]
Principal repayments	120,000,000
2018 Credit Agreement
Debt Instrument [Line Items]
Credit Agreement	$ 350,000,000
Principal repayments		$ 1,900,000
Debt instrument, frequency of periodic payment		payable quarterly
Debt instrument, payment terms of principal of debt		The 2018 Credit Agreement includes mandatory repayments of the aggregate principal amount of the 2018 Term Loan (payable quarterly) for the first, second, third, fourth, and fifth years, with the remaining principal balance due at maturity. The 2018 Credit Agreement has no prepayment penalties and requires mandatory repayments under certain circumstances, including: (i) asset sales or casualty proceeds; and (ii) proceeds of debt or preferred stock issuances.
Payments of deferred financing costs		$ 1,500,000
2018 Credit Agreement | Minimum
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage	0.20%
2018 Credit Agreement | Maximum
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage	0.375%
2018 Credit Agreement | London Interbank Offered Rate (LIBOR) | Minimum
Debt Instrument [Line Items]
Basis spread on term loan	1.50%
2018 Credit Agreement | London Interbank Offered Rate (LIBOR) | Maximum
Debt Instrument [Line Items]
Basis spread on term loan	2.50%
2018 Credit Agreement | Base Rate | Minimum
Debt Instrument [Line Items]
Basis spread on term loan	0.50%
2018 Credit Agreement | Base Rate | Maximum
Debt Instrument [Line Items]
Basis spread on term loan	1.50%
2018 Credit Agreement | Revolving Loan
Debt Instrument [Line Items]
Credit Agreement	$ 200,000,000	$ 200,000,000
Credit facility term		5 years
Line of credit facility, unused capacity, commitment fee percentage		0.20%
Credit facility, outstanding barrowings		$ 0
Credit facility, current borrowing capacity		200,000,000
2018 Credit Agreement | 2018 Term Loan
Debt Instrument [Line Items]
Carrying value of debt	150,000,000	$ 145,558,000
Term loan period		5 years
Proceeds from term loan	$ 150,000,000
Basis spread on term loan		1.50%
Term loan combined interest rate		3.83%
Financing costs		$ 2,567,000
2015 Credit Agreement | Revolving Loan
Debt Instrument [Line Items]
Credit Agreement		200,000,000
Unamortized debt issuance costs		800,000
Loss on extinguishment of debt		$ 800,000