Quarterly Financial Information (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, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Quarterly Financial Information [Abstract]
Operating revenues	$ 260,748		$ 88,195		$ 131,714		$ 277,861		$ 229,476		$ 87,501		$ 103,991		$ 309,639						$ 758,518	$ 730,607	$ 828,055
Net income (loss)	$ 29,006		$ (8,233)		$ 2,126		$ 37,639		$ 28,147	[1]	$ (10,879)	[1]	$ 1,227	[1]	$ 40,284	[1]	$ 28,766	$ (8,527)	$ 1,985	$ 40,820	$ 60,538	$ 58,779	$ 63,044
Earnings per share of common stock - basic	$ 1.07	[2]	$ (0.31)	[2]	$ 0.08	[2]	$ 1.40	[2]	$ 1.05	[1],[2]	$ (0.41)	[1],[2]	$ 0.05	[1],[2]	$ 1.50	[1],[2]	$ 1.08	$ (0.32)	$ 0.07	$ 1.53	$ 2.24	$ 2.19	$ 2.36
Earnings per share of common stock - diluted	$ 1.07	[2]	$ (0.31)	[2]	$ 0.08	[2]	$ 1.40	[2]	$ 1.04	[1],[2]	$ (0.41)	[1],[2]	$ 0.05	[1],[2]	$ 1.50	[1],[2]	$ 1.07	$ (0.32)	$ 0.07	$ 1.53	$ 2.24	$ 2.18	$ 2.36

[1] Prior period balances have been adjusted for a prior period error identified during the first quarter of 2013. See Note 16 for reconciliation to amounts previously reported.

[2] Quarterly earnings (loss) per share are based upon the average number of common shares outstanding during each quarter. Variations in earnings between quarterly periods are due primarily to the seasonal nature of our business.