Quarterly Financial Data (Unaudited) (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 Data (Unaudited)
Operating revenues	$ 804,368	$ 850,740	$ 839,130	$ 803,487	$ 810,726	$ 865,259	$ 937,214	$ 921,173	$ 3,297,725	$ 3,534,372	$ 3,654,923
Operating income	30,820	56,174	50,555	15,561	43,750	54,974	46,806	20,457	153,111	165,987	41,105
Net income (loss)	$ 8,609	$ 26,394	$ 20,720	$ 3,233	$ 13,946	$ 20,933	$ 16,960	$ (682)	$ 58,956	$ 51,157	$ (27,335)
Net income (loss) per common share:
Basic (in dollars per share)	$ 0.17	$ 0.51	$ 0.40	$ 0.06	$ 0.27	$ 0.41	$ 0.33	$ (0.01)	$ 1.14	$ 1.00	$ (0.52)
Diluted (in dollars per share)	$ 0.17	$ 0.50	$ 0.39	$ 0.06	$ 0.27	$ 0.40	$ 0.33	$ (0.01)	$ 1.12	$ 0.99	$ (0.52)
Weighted average common shares:
Basic (in shares)	51,228	51,881	51,881	51,763	51,296	51,241	50,944	50,881	51,688	51,090	52,201
Diluted (in shares)	52,034	52,610	52,547	52,497	52,161	52,153	51,789	50,881	52,422	51,746	52,201