Annual report pursuant to Section 13 and 15(d)

QUARTERLY FINANCIAL INFORMATION (UNAUDITED) (Details)

v3.8.0.1
QUARTERLY FINANCIAL INFORMATION (UNAUDITED) (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]                      
Operating revenues $ 57,531 $ 61,385 $ 57,878 $ 54,527 $ 56,289 $ 65,006 $ 63,351 $ 62,582 $ 231,321 $ 247,228 $ 281,837
Operating income (loss) (8,359) (122,773) (276) (5,056) (1,421) 2,366 (3,509) (805) (136,464) (3,369) 24,294
Net income (loss) 61,459 (81,215) (3,072) (5,787) (5,648) (802) (4,510) (3,950) $ (28,615) $ (14,910) 7,899
Net income (loss) attributable to common shares $ 61,694 $ (81,448) $ (2,787) $ (5,620) $ (5,538) $ (560) $ 1,938 $ (3,818)     $ 8,705
Earnings (loss) per common share, basic (in dollars per share) $ 2.89 $ (3.91) $ (0.13) $ (0.27) $ (0.27) $ (0.03) $ 0.09 $ (0.19) $ (1.36) $ (0.39) $ 0.42
Earnings (loss) per common share, diluted (in dollars per share) $ 2.89 $ (3.91) $ (0.13) $ (0.27) $ (0.27) $ (0.03) $ 0.09 $ (0.19) $ (1.36) $ (0.39) $ 0.42