Selected Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Information [Abstract]
Revenues	$ 378,551		$ 400,974		$ 405,431		$ 387,988		$ 359,121		$ 333,776		$ 340,421		$ 318,532		$ 1,572,944	$ 1,351,850	$ 1,315,123
Gross profit (loss)	(89,796)	[1]	103,983	[1]	113,973	[1]	115,143	[1]	92,692	[1]	79,082	[1]	90,823	[1]	83,683	[1]
Net income	$ (42,551)		$ 67,522		$ 54,360		$ 56,945		$ 51,301		$ 34,232		$ 59,007		$ 40,206		$ 136,276	$ 184,746	$ 23,176
Net income (loss) per common share:
Basic	$ (0.88)		$ 1.39		$ 1.12		$ 1.17		$ 1.06		$ 0.71		$ 1.22		$ 0.84		$ 2.80	$ 3.83	$ 0.48
Diluted	$ (0.88)	[2]	$ 1.37		$ 1.11		$ 1.17		$ 1.05		$ 0.70		$ 1.22		$ 0.83		$ 2.78	$ 3.80	$ 0.48

[1] Gross profit excludes general and administrative expense, interest expense, (gain) loss on disposition of assets, gain (loss) on non-designated hedges and hedge ineffectiveness, income taxes, and other income (loss).

[2] Due to the effect of the loss in the fourth quarter, the diluted earnings per share for the year's four quarters does not equal annual earnings per share.