Long-Term Debt - Maturities Of Long-Term Debt (Details) - USD ($) $ in Thousands		9 Months Ended
	Jun. 30, 2017	Sep. 30, 2018	Mar. 20, 2018	Dec. 31, 2017	Nov. 01, 2016
Debt Instrument [Line Items]
Less than one year		$ 46,784
One to two years		135,831
Two to three years		7,560
Three to four years		132,937
Four to five years		133,333
Over five years		32,580
Total debt		489,025
Current maturities		(46,784)		$ (109,861)
Long-term debt, less current maturities		442,241
Unamortized debt discount		(30,300)
Unamortized debt issuance costs		(10,676)
Long-term debt		401,265		385,766
Term Loan Maturing June 2020
Debt Instrument [Line Items]
Less than one year		4,212
One to two years		30,417
Two to three years		0
Three to four years		0
Four to five years		0
Over five years		0
Total debt	$ 100,000	34,629		97,500
Current maturities		(4,212)
Long-term debt, less current maturities		30,417
Unamortized debt discount		0
Unamortized debt issuance costs		(434)
Long-term debt		29,983
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		(11,772)		(13,900)	$ (16,900)
Unamortized debt issuance costs		(1,898)
Long-term debt		$ 111,330
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		125,000
Over five years		0
Total debt		125,000	$ 125,000	0
Current maturities		0
Long-term debt, less current maturities		125,000
Unamortized debt discount		(18,528)	$ (20,100)
Unamortized debt issuance costs		(2,986)
Long-term debt		$ 103,486
Maturity date		September 2023
MARAD Debt Maturing February 2027
Debt Instrument [Line Items]
Less than one year		$ 6,858
One to two years		7,200
Two to three years		7,560
Three to four years		7,937
Four to five years		8,333
Over five years		32,580
Total debt		70,468		77,000
Current maturities		(6,858)
Long-term debt, less current maturities		63,610
Unamortized debt discount		0
Unamortized debt issuance costs		(4,147)
Long-term debt		$ 59,463
Maturity date		February 2027
Nordea Q5000 Loan Maturing April 2020
Debt Instrument [Line Items]
Less than one year		$ 35,714
One to two years		98,214
Two to three years		0
Three to four years		0
Four to five years		0
Over five years		0
Total debt		133,928		$ 160,714
Current maturities		(35,714)
Long-term debt, less current maturities		98,214
Unamortized debt discount		0
Unamortized debt issuance costs		(1,211)
Long-term debt		$ 97,003