Quarterly Financial Data - Schedule of Quarterly Financial Information (Detail) (USD $) In Thousands, except Per Share data, 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
Interim Reporting [Line Items]
Operating Revenue	$ 109,516	$ 78,175	$ 83,897	$ 120,914	$ 103,988	$ 80,610	$ 86,831	$ 146,597	$ 392,502	$ 418,027	$ 427,546
Operating Income	18,543	7,564	10,455	20,073	15,495	5,594	7,776	24,839	56,635	53,705	51,930
Net Income	$ 9,857	$ 3,219	$ 5,060	$ 10,727	$ 7,957	$ 2,397	$ 3,520	$ 13,747	$ 28,863	$ 27,622	$ 26,056
Earnings per share:
Basic	$ 1.03	$ 0.34	$ 0.53	$ 1.12	$ 0.83	$ 0.25	$ 0.37	$ 1.44	$ 3.01	$ 2.89	$ 2.75
Diluted	$ 1.02	$ 0.33	$ 0.52	$ 1.11	$ 0.83	$ 0.25	$ 0.37	$ 1.43	$ 2.99	$ 2.87	$ 2.73