Long-Term Debt - Nordea Credit Agreement (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Apr. 30, 2015	Sep. 30, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 30, 2015
Debt Instrument [Line Items]
Funded amount			$ 0	$ 0	$ 250,000
Nordea Q5000 Loan Maturing April 2020 [Member]
Debt Instrument [Line Items]
Borrowing capacity		$ 250,000
Funded amount	$ 250,000
Maturity date		Apr. 30, 2020
Frequency of periodic payment		quarterly
Scheduled principal installments		$ 8,900
Balloon payment		$ 80,400
Nordea Q5000 Loan Maturing April 2020 [Member] | Interest Rate Swaps [Member]
Debt Instrument [Line Items]
Notional amount						$ 187,500
Fixed LIBOR rate on interest rate swaps (as a percent)						1.50%
Nordea Q5000 Loan Maturing April 2020 [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)		2.50%