CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income (loss)		$ 100,393	$ (417,124)	$ (113,891)
Postretirement benefits:
Net gain (loss)	[1]	(2,634)	1,794	1,786
Prior service credit (cost)	[2]	5,449	(514)	0
Amortization of prior service cost	[3]	(1,723)	70	0
Total other comprehensive income		1,092	1,350	1,786
Comprehensive income (loss)		$ 101,485	$ (415,774)	$ (112,105)

[1] Net of income taxes of $1,544, $(1,052) and $(1,043) for the year ended December 31, 2017, 2016 and 2015, respectively.

[2] Net of income taxes of $(3,194) and $301 for the year ended December 31, 2017 and 2016, respectively.

[3] Net of income taxes of $1,010 and $(41) for the year ended December 31, 2017 and 2016, respectively.