Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Quarterly Financial Information [Abstract]
Operating revenues	$ 230,718		$ 93,128		$ 138,280		$ 261,665		$ 240,283		$ 87,199		$ 133,169		$ 293,386		$ 723,791	$ 754,037	$ 758,518
Net Income	$ 29,705		$ (6,685)		$ 2,197		$ 28,486		$ 28,470		$ (8,733)		$ 1,071		$ 37,884		$ 53,703	$ 58,692	$ 60,538
Earnings per share of common stock - basic	$ 1.08	[1]	$ (0.24)	[1]	$ 0.08	[1]	$ 1.04	[1]	$ 1.05	[1]	$ (0.32)	[1]	$ 0.04	[1]	$ 1.40	[1]	$ 1.96	$ 2.16	$ 2.24
Earnings per share of common stock - diluted	$ 1.08	[1]	$ (0.24)	[1]	$ 0.08	[1]	$ 1.04	[1]	$ 1.04	[1]	$ (0.32)	[1]	$ 0.04	[1]	$ 1.40	[1]	$ 1.96	$ 2.16	$ 2.24

[1] 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.