Long-Term Debt - Maturities Of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jun. 28, 2019	Dec. 31, 2018	Mar. 20, 2018	Nov. 01, 2016
Debt Instrument [Line Items]
Less than one year	$ 117,670
One to two years	10,878
Two to three years	160,746
Three to four years	8,133
Four to five years	133,539
Over five years	28,258
Gross debt	459,224
Unamortized debt discounts	(25,732)
Unamortized debt issuance costs	(9,004)
Total debt	424,488
Less: current maturities	(117,033)		$ (47,252)
Long-term debt	307,455		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	28,000
Three to four years	0
Four to five years	0
Over five years	0
Gross debt	35,000	$ 35,000	0
Unamortized debt discounts	0
Unamortized debt issuance costs	(477)
Total debt	34,523
Less: current maturities	(3,500)
Long-term debt	31,023
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	(9,549)		(11,000)		$ (16,900)
Unamortized debt issuance costs	(1,501)
Total debt	113,950
Less: current maturities	0
Long-term debt	113,950
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	$ 125,000
Unamortized debt discounts	(16,183)		(17,800)	$ (20,100)
Unamortized debt issuance costs	(2,608)
Total debt	106,209
Less: current maturities	0
Long-term debt	106,209
MARAD Debt Maturing February 2027
Debt Instrument [Line Items]
Less than one year	7,027
One to two years	7,378
Two to three years	7,746
Three to four years	8,133
Four to five years	8,539
Over five years	28,258
Gross debt	67,081		70,468
Unamortized debt discounts	0
Unamortized debt issuance costs	(3,781)
Total debt	63,300
Less: current maturities	(7,027)
Long-term debt	56,273
Nordea Q5000 Loan Maturing April 2020
Debt Instrument [Line Items]
Less than one year	107,143
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	107,143		$ 125,000
Unamortized debt discounts	0
Unamortized debt issuance costs	(637)
Total debt	106,506
Less: current maturities	(106,506)
Long-term debt	$ 0