Long-Term Debt - Maturities Of Long-Term Debt (Details) - USD ($) $ in Thousands		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2018	Mar. 20, 2018	Dec. 31, 2017	Nov. 01, 2016
Debt Instrument [Line Items]
Less than one year		$ 46,151
One to two years		145,990
Two to three years		7,378
Three to four years		132,746
Four to five years		8,133
Over five years		161,797
Total debt		502,195
Current maturities		(46,151)		$ (109,861)
Long-term debt, less current maturities		456,044
Unamortized debt discount		(31,878)
Unamortized debt issuance costs		(11,314)
Long-term debt		412,852		385,766
Term Loan Maturing June 2020
Debt Instrument [Line Items]
Less than one year		3,744
One to two years		31,820
Two to three years		0
Three to four years		0
Four to five years		0
Over five years		0
Total debt	$ 100,000	35,564		97,500
Current maturities		(3,744)
Long-term debt, less current maturities		31,820
Unamortized debt discount		0
Unamortized debt issuance costs		(496)
Long-term debt		31,324
Maturity date	June 2020
Convertible Senior Notes Maturing May 2022
Debt Instrument [Line Items]
Less than one year		0
One to two years		0
Two to three years		0
Three to four years		125,000
Four to five years		0
Over five years		0
Total debt		125,000		125,000	$ 125,000
Current maturities		0
Long-term debt, less current maturities		125,000
Unamortized debt discount		(12,484)		(13,900)	$ (16,900)
Unamortized debt issuance costs		(2,030)
Long-term debt		$ 110,486
Maturity date		May 2022
Convertible Senior Notes Maturing September 2023
Debt Instrument [Line Items]
Less than one year		$ 0
One to two years		0
Two to three years		0
Three to four years		0
Four to five years		0
Over five years		125,000
Total debt		125,000	$ 125,000	0
Current maturities		0
Long-term debt, less current maturities		125,000
Unamortized debt discount		(19,394)	$ (20,100)
Unamortized debt issuance costs		(3,117)
Long-term debt		$ 102,489
Maturity date		September 2023
MARAD Debt Maturing February 2027
Debt Instrument [Line Items]
Less than one year		$ 6,693
One to two years		7,027
Two to three years		7,378
Three to four years		7,746
Four to five years		8,133
Over five years		36,797
Total debt		73,774		77,000
Current maturities		(6,693)
Long-term debt, less current maturities		67,081
Unamortized debt discount		0
Unamortized debt issuance costs		(4,269)
Long-term debt		$ 62,812
Maturity date		February 2027
Nordea Q5000 Loan Maturing April 2020
Debt Instrument [Line Items]
Less than one year		$ 35,714
One to two years		107,143
Two to three years		0
Three to four years		0
Four to five years		0
Over five years		0
Total debt		142,857		$ 160,714
Current maturities		(35,714)
Long-term debt, less current maturities		107,143
Unamortized debt discount		0
Unamortized debt issuance costs		(1,402)
Long-term debt		$ 105,741