Long-Term Debt - Maturities Of Long-Term Debt (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016	Nov. 01, 2016	Mar. 31, 2012
Debt Instrument [Line Items]
Less than one year	$ 109,861
One to two years	55,072
Two to three years	173,986
Three to four years	7,560
Four to five years	132,937
Over five years	40,913
Total debt	520,329
Less current maturities	(109,861)		$ (67,571)
Long-term debt, less current maturities	410,468
Unamortized debt discounts	(14,406)		(19,094)
Unamortized debt issuance costs	(10,296)		(11,963)
Long-term debt	385,766		558,396
Term Loan Maturing June 2020 [Member]
Debt Instrument [Line Items]
Less than one year	7,500
One to two years	12,500
Two to three years	77,500
Three to four years	0
Four to five years	0
Over five years	0
Total debt	97,500	$ 100,000	0
Less current maturities	(7,500)
Long-term debt, less current maturities	90,000
Unamortized debt discounts	0
Unamortized debt issuance costs	(1,658)
Long-term debt	88,342
Convertible Senior Notes Maturing May 2022 [Member]
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
Less current maturities	0
Long-term debt, less current maturities	125,000
Unamortized debt discounts	(13,876)		(16,500)	$ (16,900)
Unamortized debt issuance costs	(2,295)
Long-term debt	108,829
Convertible Senior Notes Maturing March 2032 [Member]
Debt Instrument [Line Items]
Less than one year	60,115
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	60,115		60,115		$ 200,000
Less current maturities	(60,115)
Long-term debt, less current maturities	0
Unamortized debt discounts	(530)		(2,600)		$ (35,400)
Unamortized debt issuance costs	(46)
Long-term debt	$ (576)
Earliest put date	March 2018
MARAD Debt Maturing February 2027 [Member]
Debt Instrument [Line Items]
Less than one year	$ 6,532
One to two years	6,858
Two to three years	7,200
Three to four years	7,560
Four to five years	7,937
Over five years	40,913
Total debt	77,000		83,222
Less current maturities	(6,532)
Long-term debt, less current maturities	70,468
Unamortized debt discounts	0
Unamortized debt issuance costs	(4,513)
Long-term debt	65,955
Nordea Q5000 Loan Maturing April 2020 [Member]
Debt Instrument [Line Items]
Less than one year	35,714
One to two years	35,714
Two to three years	89,286
Three to four years	0
Four to five years	0
Over five years	0
Total debt	160,714		$ 196,429
Less current maturities	(35,714)
Long-term debt, less current maturities	125,000
Unamortized debt discounts	0
Unamortized debt issuance costs	(1,784)
Long-term debt	$ 123,216