Long-Term Debt - Maturities Of Long-Term Debt (Details) - USD ($) $ in Thousands		3 Months Ended
	Jun. 30, 2017	Mar. 31, 2018	Mar. 20, 2018	Dec. 31, 2017	Nov. 01, 2016	Mar. 31, 2012
Debt Instrument [Line Items]
Less than one year		$ 46,492
One to two years		47,888
Two to three years		115,344
Three to four years		7,746
Four to five years		133,133
Over five years		161,797
Total debt		512,400
Current maturities		(46,492)		$ (109,861)
Long-term debt, less current maturities		465,908
Unamortized debt discount		(33,177)
Unamortized debt issuance costs		(11,853)
Long-term debt		420,878		385,766
Term Loan Maturing June 2020
Debt Instrument [Line Items]
Less than one year		3,276
One to two years		5,147
Two to three years		27,609
Three to four years		0
Four to five years		0
Over five years		0
Total debt	$ 100,000	36,032		97,500
Current maturities		(3,276)
Long-term debt, less current maturities		32,756
Unamortized debt discount		0
Unamortized debt issuance costs		(559)
Long-term debt		32,197
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		0
Four to five years		125,000
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		(13,188)		(13,900)	$ (16,900)
Unamortized debt issuance costs		(2,162)
Long-term debt		$ 109,650
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		(19,989)	$ (20,100)
Unamortized debt issuance costs		(3,148)
Long-term debt		$ 101,863
Maturity date		September 2023
Convertible Senior Notes Maturing March 2032
Debt Instrument [Line Items]
Less than one year		$ 809
One to two years		0
Two to three years		0
Three to four years		0
Four to five years		0
Over five years		0
Total debt		809		60,115		$ 200,000
Current maturities		(809)
Long-term debt, less current maturities		0
Unamortized debt discount		0		(500)		$ (35,400)
Unamortized debt issuance costs		0
Long-term debt		0
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		(4,391)
Long-term debt		$ 62,690
Maturity date		February 2027
Nordea Q5000 Loan Maturing April 2020
Debt Instrument [Line Items]
Less than one year		$ 35,714
One to two years		35,714
Two to three years		80,357
Three to four years		0
Four to five years		0
Over five years		0
Total debt		151,785		$ 160,714
Current maturities		(35,714)
Long-term debt, less current maturities		116,071
Unamortized debt discount		0
Unamortized debt issuance costs		(1,593)
Long-term debt		$ 114,478