Summarized Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, 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	$ 240,388	$ 231,060	$ 210,175	$ 241,415	$ 217,395	$ 226,475	$ 194,173	$ 223,799	$ 923,038	$ 861,842	$ 875,790
Operating income	16,545	40,769	22,466	37,401	15,869	40,735	14,348	28,914	117,181	99,866	107,508
Net income	$ 7,225	$ 22,013	$ 10,544	$ 19,685	$ 7,131	$ 22,200	$ 5,742	$ 14,878	$ 59,468	$ 49,951	$ 51,006