CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income (loss)		$ (113,891)	$ 104,468	$ 279,773
Other comprehensive income (loss), net of taxes:
Reclassification adjustment for settled cash flow hedge contracts	[1]	0	86,726	(34,548)
Changes in fair value of cash flow hedge contracts	[2]	0	(80,175)	(2,720)
Postretirement benefits:
Net gain (loss)	[3]	1,786	(325)	4,641
Amortization of net (gain) loss	[4]	0	(16)	386
Total other comprehensive income (loss)		1,786	6,210	(32,241)
Comprehensive income (loss)		$ (112,105)	$ 110,678	$ 247,532

[1] Net of income taxes of $(57,477) and $22,454 for the year ended December 31, 2014 and 2013, respectively.

[2] Net of income taxes of $53,135 and $1,803 for the year ended December 31, 2014 and 2013, respectively.

[3] Net of income taxes of $(1,043), $48 and $(2,977) for the year ended December 31, 2015, 2014 and 2013, respectively.

[4] Net of income taxes of $10 and $(255) for the year ended December 31, 2014 and 2013, respectively.