Quarterly Results of Operations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 509,694	$ 506,504	$ 521,812	$ 498,376	$ 507,937	$ 509,587	$ 515,897	$ 469,429	$ 2,036,386	$ 2,002,850	$ 1,815,020
Operating income	43,124	41,805	51,113	35,402	49,399	50,066	46,767	27,442	171,444	173,674	134,582
Net Income	$ 25,981	$ 25,128	$ 30,680	$ 21,245	$ 29,368	$ 29,578	$ 27,518	$ 16,293	$ 103,034	$ 102,757	$ 80,039
Basic earnings per share	$ 0.36	$ 0.34	$ 0.42	$ 0.29	$ 0.40	$ 0.41	$ 0.38	$ 0.22	$ 1.41	$ 1.41	$ 1.11
Diluted earnings per share	$ 0.35	$ 0.34	$ 0.42	$ 0.29	$ 0.40	$ 0.40	$ 0.38	$ 0.22	$ 1.40	$ 1.40	$ 1.10