Fair Value Measurements - Carrying Values And Estimated Fair Values Of Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 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	$ 530,508
Term Loan Maturing June 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	98,750	$ 100,000	$ 0
Fair value	99,120		0
Term Loan Maturing June 2018 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	0		192,258
Fair value	0		192,258
Nordea Q5000 Loan Maturing April 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	169,643		196,429
Fair value	168,583		192,746
MARAD Debt Maturing February 2027 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	77,000		83,222
Fair value	83,928		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	123,281		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	60,077		59,965
Loan Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	530,508		657,024
Fair value	$ 534,989		$ 667,174