Long-Term Debt - Maturities Of Long-Term Debt (Details) - USD ($) $ in Thousands		1 Months Ended	6 Months Ended
	Jun. 30, 2017	Aug. 31, 2002	Jun. 30, 2017	Dec. 31, 2016	Nov. 01, 2016	Mar. 31, 2012
Debt Instrument [Line Items]
Less than one year	$ 107,205		$ 107,205
One to two years	52,407		52,407
Two to three years	199,169		199,169
Three to four years	7,378		7,378
Four to five years	7,746		7,746
Over five years	169,930		169,930
Total debt	543,835		543,835
Current maturities	(107,205)		(107,205)	$ (67,571)
Long-term debt, less current maturities	436,630		436,630
Unamortized debt discount	(16,791)		(16,791)
Unamortized debt issuance costs	(11,589)		(11,589)
Long-term debt	408,250		408,250	558,396
Term Loan Maturing June 2020 [Member]
Debt Instrument [Line Items]
Less than one year	5,000		5,000
One to two years	10,000		10,000
Two to three years	85,000		85,000
Three to four years	0		0
Four to five years	0		0
Over five years	0		0
Total debt	100,000		100,000	0
Current maturities	(5,000)		(5,000)
Long-term debt, less current maturities	95,000		95,000
Unamortized debt discount	0		0
Unamortized debt issuance costs	(1,967)		(1,967)
Long-term debt	$ 93,033		93,033
Maturity date	June 2020
Convertible Senior Notes Maturing May 2022 [Member]
Debt Instrument [Line Items]
Less than one year	$ 0		0
One to two years	0		0
Two to three years	0		0
Three to four years	0		0
Four to five years	0		0
Over five years	125,000		125,000
Total debt	125,000		125,000	125,000	$ 125,000
Current maturities	0		0
Long-term debt, less current maturities	125,000		125,000
Unamortized debt discount	(15,218)		(15,218)	(16,500)	$ (16,900)
Unamortized debt issuance costs	(2,560)		(2,560)
Long-term debt	107,222		$ 107,222
Maturity date			May 2022
Convertible Senior Notes Maturing March 2032 [Member]
Debt Instrument [Line Items]
Less than one year	60,115		$ 60,115
One to two years	0		0
Two to three years	0		0
Three to four years	0		0
Four to five years	0		0
Over five years	0		0
Total debt	60,115		60,115	60,115		$ 200,000
Current maturities	(60,115)		(60,115)
Long-term debt, less current maturities	0		0
Unamortized debt discount	(1,573)		(1,573)	(2,600)		$ (35,400)
Unamortized debt issuance costs	(138)		(138)
Long-term debt	(1,711)		$ (1,711)
Earliest put date			March 2018
MARAD Debt Maturing February 2027 [Member]
Debt Instrument [Line Items]
Less than one year	6,375		$ 6,375
One to two years	6,693		6,693
Two to three years	7,027		7,027
Three to four years	7,378		7,378
Four to five years	7,746		7,746
Over five years	44,930		44,930
Total debt	80,149		80,149	83,222
Current maturities	(6,375)		(6,375)
Long-term debt, less current maturities	73,774		73,774
Unamortized debt discount	0		0
Unamortized debt issuance costs	(4,757)		(4,757)
Long-term debt	69,017		69,017
Maturity date		February 2027
Nordea Q5000 Loan Maturing April 2020 [Member]
Debt Instrument [Line Items]
Less than one year	35,715		35,715
One to two years	35,714		35,714
Two to three years	107,142		107,142
Three to four years	0		0
Four to five years	0		0
Over five years	0		0
Total debt	178,571		178,571	$ 196,429
Current maturities	(35,715)		(35,715)
Long-term debt, less current maturities	142,856		142,856
Unamortized debt discount	0		0
Unamortized debt issuance costs	(2,167)		(2,167)
Long-term debt	$ 140,689		$ 140,689