Summarized Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summarized Quarterly Financial Data (Unaudited) [Abstract]
Operating revenues	$ 217,395	$ 226,475	$ 194,173	$ 223,799	$ 214,364	$ 224,468	$ 198,850	$ 238,108	$ 861,842	$ 875,790	$ 827,291
Operating income	15,869	40,735	14,348	28,914	18,464	39,032	20,115	29,897	99,866	107,508	88,821
Net income	$ 7,131	$ 22,200	$ 5,742	$ 14,878	$ 7,912	$ 19,973	$ 8,478	$ 14,643	$ 49,951	$ 51,006	$ 42,749