CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income (loss)		$ (417,124)	$ (113,891)	$ 104,468
Other comprehensive income, net of taxes:
Reclassification adjustment for settled cash flow hedge contracts	[1]	0	0	86,726
Changes in fair value of cash flow hedge contracts	[2]	0	0	(80,175)
Postretirement benefits:
Net gain (loss)	[3]	1,794	1,786	(325)
Prior service cost	[4]	(514)	0	0
Amortization of prior service cost	[5]	70	0	0
Amortization of net (gain) loss	[6]	0	0	(16)
Total other comprehensive income		1,350	1,786	6,210
Comprehensive income (loss)		$ (415,774)	$ (112,105)	$ 110,678

[1] Net of income taxes of $(57,477) for the year ended December 31, 2014.

[2] Net of income taxes of $53,135 for the year ended December 31, 2014.

[3] Net of income taxes of $(1,052), $(1,043) and $48 for the year ended December 31, 2016, 2015 and 2014, respectively.

[4] Net of income taxes of $301 for the year ended December 31, 2016.

[5] Net of income taxes of $(41) for the year ended December 31, 2016.

[6] Net of income taxes of $10 for the year ended December 31, 2014.