QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended								12 Months Ended
	May 07, 2015	Dec. 31, 2016 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares	Mar. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) $ / shares shares	Mar. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares
Voyage revenues | $		$ 102,361	$ 72,259	$ 105,958	$ 124,044	$ 102,760	$ 89,291	$ 116,480	$ 121,402	$ 404,622	$ 429,933	$ 392,409
Operating income (loss) | $		23,330	(25,192)	49,948	68,184	50,408	33,490	29,436	38,665	116,270	151,999	(17,442)
Net income (loss) | $		$ 5,804	$ (37,351)	$ 37,995	$ 60,858	$ 45,521	$ 33,228	$ 19,901	$ 30,919	$ 67,306	$ 129,569	$ (47,084)
Income (loss) per common share
Basic (in dollars per share) | $ / shares		$ 0.07	$ (0.45)	$ 0.46	$ 0.74	$ 0.55	$ 0.41	$ 0.38	$ 0.93	$ 0.81	$ 2.06	$ (1.54)
Diluted (in dollars per share) | $ / shares		$ 0.07	$ (0.45)	$ 0.46	$ 0.74	$ 0.55	$ 0.40	$ 0.38	$ 0.93	$ 0.81	$ 2.05	$ (1.54)
Weighted average shares outstanding - basic
Weighted-average shares outstanding used in basic computation | shares		82,776	82,682	82,681	82,680	82,280	81,758	52,919		82,705	62,779
Weighted average shares outstanding - diluted
Weighted-average shares outstanding, diluted | shares		82,776	82,682	82,681	82,680	82,778	82,480	52,940		82,705	63,113
Class A common stock
Net income (loss) | $												$ (17,402)
Income (loss) per common share
Basic (in dollars per share) | $ / shares												$ (1.54)
Diluted (in dollars per share) | $ / shares												$ (1.54)
Weighted average shares outstanding - basic
Weighted-average shares outstanding used in basic computation | shares									11,270			11,270
Weighted average shares outstanding - diluted
Weighted-average shares outstanding, diluted | shares									33,273			30,493
Number of shares of common stock converted to single class of shares	1
Class B common stock
Net income (loss) | $												$ (29,682)
Income (loss) per common share
Basic (in dollars per share) | $ / shares												$ (1.54)
Diluted (in dollars per share) | $ / shares												$ (1.54)
Weighted average shares outstanding - basic
Weighted-average shares outstanding used in basic computation | shares									22,003			19,223
Weighted average shares outstanding - diluted
Weighted-average shares outstanding, diluted | shares									22,003			19,223
Number of shares of common stock converted to single class of shares	1