Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Data (Unaudited)
Operating revenues	$ 752,744	$ 794,004	$ 788,417	$ 760,398	$ 813,854	$ 834,633	$ 816,574	$ 772,386	$ 3,095,563	$ 3,237,447	$ 3,297,725
Operating income	52,212	78,296	69,932	34,075	(19,702)	59,080	13,244	(27,774)	234,515	24,848	153,111
Net Income (loss)	$ 40,454	$ 36,268	$ 31,475	$ 9,620	$ (27,868)	$ 41,338	$ (14,737)	$ (22,887)	$ 117,817	$ (24,154)	$ 58,956
Net income (loss) per common share:
Basic (in dollars per share)	$ 0.80	$ 0.72	$ 0.61	$ 0.19	$ (0.54)	$ 0.81	$ (0.29)	$ (0.44)	$ 2.31	$ (0.47)	$ 1.14
Diluted (in dollars per share)	$ 0.78	$ 0.71	$ 0.61	$ 0.18	$ (0.54)	$ 0.79	$ (0.29)	$ (0.44)	$ 2.27	$ (0.47)	$ 1.12
Weighted average common shares:
Basic (in shares)	50,880	50,616	51,357	51,457	51,174	51,127	51,183	51,467	51,077	51,237	51,688
Diluted (in shares)	51,657	51,282	51,971	52,392	51,174	52,036	51,183	51,467	51,825	51,237	52,422