Fair Value Measurements - Principal Amount And Estimated Fair Value Of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 20, 2018	Dec. 31, 2017	Jun. 30, 2017	Nov. 01, 2016	Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	$ 502,195
Term Loan Maturing June 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	35,564		$ 97,500	$ 100,000
Fair value	35,631		98,231
Nordea Q5000 Loan Maturing April 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	142,857		160,714
Fair value	142,054		160,111
MARAD Debt Maturing February 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	73,774		77,000
Fair value	77,140		82,058
Convertible Senior Notes Maturing May 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	125,000		125,000		$ 125,000
Fair value	129,219		124,219
Convertible Senior Notes Maturing September 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	125,000	$ 125,000	0
Fair value	147,812		0
Convertible Senior Notes Maturing March 2032 (Redeemed May 2018)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	0		60,115			$ 200,000
Fair value	0		60,040
Total Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	502,195		520,329
Fair value	$ 531,856		$ 524,659