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	$ 347,477	$ 471,889	$ 380,201	$ 340,488	$ 370,146	$ 522,921	$ 433,289	$ 381,209	$ 1,540,055	$ 1,707,565	$ 1,612,990
Operating income	24,510	107,509	61,891	31,845	24,780	92,885	53,433	29,499	225,755	200,597	183,437
Net income	$ 6,468	$ 58,247	$ 30,294	$ 11,360	$ 6,410	$ 48,581	$ 24,672	$ 10,238	$ 106,369	$ 89,901	$ 78,067