Selected Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010		Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Selected Quarterly Financial Information [Abstract]
Revenues	$ 345,626		$ 323,845	$ 291,495	$ 247,405	$ 252,576		$ 218,116	$ 204,603	$ 206,550	$ 1,208,371	$ 881,845	$ 709,898
Gross profit	94,745	[1]	96,581	89,164	73,568	77,046	[1]	63,371	53,499	59,319
Net income (loss)	$ 51,661		$ 53,360	$ 49,819	$ 41,027	$ 43,665		$ 34,491	$ 32,175	$ 36,153	$ 195,867	$ 146,484	$ (55,500)
Basic	$ 1.08		$ 1.12	$ 1.05	$ 0.86	$ 0.92		$ 0.73	$ 0.68	$ 0.77	$ 4.11	$ 3.10	$ (1.18)
Diluted	$ 1.08	[2]	$ 1.11	$ 1.04	$ 0.86	$ 0.92		$ 0.73	$ 0.68	$ 0.76	$ 4.08	$ 3.09	$ (1.18)

[1] Gross profit excludes other revenues, general and administrative expense and interest expense.

[2] Due to the effect of rounding the basic earnings or diluted per share for the year's four quarters does not equal annual earnings per share.