Quarterly Results of Operations (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
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 517,920	$ 511,728	$ 506,648	$ 492,887	$ 509,694	$ 506,504	$ 521,812	$ 498,376	$ 2,029,183	$ 2,036,386	$ 2,002,850
Operating income	36,089	32,583	42,361	28,693	43,124	41,805	51,113	35,402	139,726	171,444	173,674
Net Income	$ 22,175	$ 21,259	$ 25,840	$ 17,511	$ 25,981	$ 25,128	$ 30,680	$ 21,245	$ 86,785	$ 103,034	$ 102,757
Basic earnings per share	$ 0.31	$ 0.29	$ 0.35	$ 0.24	$ 0.36	$ 0.34	$ 0.42	$ 0.29	$ 1.19	$ 1.41	$ 1.41
Diluted earnings per share	$ 0.30	$ 0.29	$ 0.35	$ 0.24	$ 0.35	$ 0.34	$ 0.42	$ 0.29	$ 1.18	$ 1.40	$ 1.40