Fair Value Measurements - Carrying Values And Estimated Fair Values Of Long-Term Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Nov. 01, 2016	Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	$ 638,613
Term Loan Maturing June 2018 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	185,849	$ 192,258
Fair value	187,475	192,258
Nordea Q5000 Loan Maturing April 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	187,500	196,429
Fair value	184,806	192,746
MARAD Debt Maturing February 2027 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	80,149	83,222
Fair value	93,644	92,049
Convertible Senior Notes Maturing May 2022 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	125,000	125,000	$ 125,000
Fair value	123,750	130,156
Convertible Senior Notes Maturing March 2032 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	60,115	60,115		$ 200,000
Fair value	60,265	59,965
Loan Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	638,613	657,024
Fair value	$ 649,940	$ 667,174