Long-Term Debt - Maturities Of Long-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Mar. 20, 2018	Dec. 31, 2017	Jun. 30, 2017	Nov. 01, 2016
Debt Instrument [Line Items]
Less than one year	$ 47,252
One to two years	125,499
Two to three years	7,560
Three to four years	132,937
Four to five years	133,333
Over five years	32,580
Gross debt	479,161
Unamortized debt discounts	(28,802)		$ (14,406)
Unamortized debt issuance costs	(10,044)		(10,296)
Total debt	440,315		495,627
Less current maturities	(47,252)		(109,861)
Long-term debt	393,063		385,766
Term Loan Maturing June 2020
Debt Instrument [Line Items]
Less than one year	4,680
One to two years	29,013
Two to three years	0
Three to four years	0
Four to five years	0
Over five years	0
Gross debt	33,693		97,500	$ 100,000
Unamortized debt discounts	0
Unamortized debt issuance costs	(372)
Total debt	33,321
Less current maturities	(4,680)
Long-term debt	28,641
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
Gross debt	125,000		125,000		$ 125,000
Unamortized debt discounts	(11,043)		(13,900)		$ (16,900)
Unamortized debt issuance costs	(1,765)
Total debt	112,192
Less current maturities	0
Long-term debt	112,192
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
Gross debt	125,000	$ 125,000	0
Unamortized debt discounts	(17,759)	$ (20,100)
Unamortized debt issuance costs	(2,862)
Total debt	104,379
Less current maturities	0
Long-term debt	104,379
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
Gross debt	70,468		77,000
Unamortized debt discounts	0
Unamortized debt issuance costs	(4,025)
Total debt	66,443
Less current maturities	(6,858)
Long-term debt	59,585
Nordea Q5000 Loan Maturing April 2020
Debt Instrument [Line Items]
Less than one year	35,714
One to two years	89,286
Two to three years	0
Three to four years	0
Four to five years	0
Over five years	0
Gross debt	125,000		$ 160,714
Unamortized debt discounts	0
Unamortized debt issuance costs	(1,020)
Total debt	123,980
Less current maturities	(35,714)
Long-term debt	$ 88,266