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Long-Term Debt - Maturities Of Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 28, 2019	Dec. 31, 2018	Mar. 20, 2018	Nov. 01, 2016
Debt Instrument [Line Items]
Less than one year	$ 108,914
One to two years	11,060
Two to three years	160,062
Three to four years	133,333
Four to five years	8,749
Over five years	23,831
Gross debt	445,949
Unamortized debt discounts	(24,160)
Unamortized debt issuance costs	(8,389)
Total debt	413,400
Less: current maturities	(108,468)		$ (47,252)
Long-term debt	304,932		393,063
Term Loan Maturing December 2021
Debt Instrument [Line Items]
Less than one year	3,500
One to two years	3,500
Two to three years	27,125
Three to four years	0
Four to five years	0
Over five years	0
Gross debt	34,125	$ 35,000	0
Unamortized debt discounts	0
Unamortized debt issuance costs	(438)
Total debt	33,687
Less: current maturities	(3,500)
Long-term debt	30,187
Convertible Senior Notes Maturing May 2022
Debt Instrument [Line Items]
Less than one year	0
One to two years	0
Two to three years	125,000
Three to four years	0
Four to five years	0
Over five years	0
Gross debt	125,000		125,000		$ 125,000
Unamortized debt discounts	(8,784)		(11,000)		$ (16,900)
Unamortized debt issuance costs	(1,368)
Total debt	114,848
Less: current maturities	0
Long-term debt	114,848
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	125,000
Four to five years	0
Over five years	0
Gross debt	125,000		125,000	$ 125,000
Unamortized debt discounts	(15,376)		(17,800)	$ (20,100)
Unamortized debt issuance costs	(2,478)
Total debt	107,146
Less: current maturities	0
Long-term debt	107,146
MARAD Debt Maturing February 2027
Debt Instrument [Line Items]
Less than one year	7,200
One to two years	7,560
Two to three years	7,937
Three to four years	8,333
Four to five years	8,749
Over five years	23,831
Gross debt	63,610		70,468
Unamortized debt discounts	0
Unamortized debt issuance costs	(3,659)
Total debt	59,951
Less: current maturities	(7,200)
Long-term debt	52,751
Nordea Q5000 Loan Maturing April 2020
Debt Instrument [Line Items]
Less than one year	98,214
One to two years	0
Two to three years	0
Three to four years	0
Four to five years	0
Over five years	0
Gross debt	98,214		$ 125,000
Unamortized debt discounts	0
Unamortized debt issuance costs	(446)
Total debt	97,768
Less: current maturities	(97,768)
Long-term debt	$ 0