Quarterly Results of Operations (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
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 553,186	$ 551,961	$ 542,120	$ 492,022	$ 517,920	$ 511,728	$ 506,648	$ 492,887	$ 2,139,289	$ 2,029,183	$ 2,036,386
Operating income	52,627	41,690	42,330	23,441	36,089	32,583	42,361	28,693	160,088	139,726	171,444
Net Income	$ 32,709	$ 25,970	$ 25,632	$ 14,339	$ 22,175	$ 21,259	$ 25,840	$ 17,511	$ 98,650	$ 86,785	$ 103,034
Basic earnings per share	$ 0.45	$ 0.36	$ 0.36	$ 0.20	$ 0.31	$ 0.29	$ 0.35	$ 0.24	$ 1.37	$ 1.19	$ 1.41
Diluted earnings per share	$ 0.45	$ 0.36	$ 0.35	$ 0.20	$ 0.30	$ 0.29	$ 0.35	$ 0.24	$ 1.36	$ 1.18	$ 1.40