Selected Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Selected Quarterly Financial Information [Abstract]
Revenues	$ 359,121		$ 333,776		$ 340,421		$ 318,532		$ 331,582		$ 321,790		$ 327,785		$ 333,966		$ 1,351,850	$ 1,315,123	$ 1,207,503
Gross profit	92,692	[1]	79,082	[1]	90,823	[1]	83,683	[1]	(81,833)	[1]	95,921	[1]	(22,253)	[1]	95,912	[1]
Net income	$ 51,301		$ 34,232		$ 59,007		$ 40,206		$ (56,547)		$ 46,586		$ (19,302)		$ 52,439		$ 184,746	$ 23,176	$ 195,867
Basic	$ 1.06		$ 0.71		$ 1.22		$ 0.84		$ (1.18)	[2]	$ 0.97	[2]	$ (0.40)	[2]	$ 1.10	[2]	$ 3.83	$ 0.48	$ 4.11
Diluted	$ 1.05		$ 0.70		$ 1.22		$ 0.83		$ (1.18)		$ 0.97		$ (0.40)		$ 1.09		$ 3.80	$ 0.48	$ 4.08

[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 rounding the basic earnings per share for the year’s four quarters does not equal annual earnings per share.