Quarterly Financial Data - Summary of Condensed Quarterly Results of Operations (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 210,760	$ 216,294	$ 212,145	$ 197,071	$ 281,227	$ 200,762	$ 195,543	$ 178,968	$ 836,270	$ 856,500	$ 646,686
Gain (loss) on disposition of assets, net	523	(56)	(242)	(1,073)	(529)	(1,399)	(14,701)	25	(848)	16,604	(317)
Earnings (loss) before income taxes	11,651	28,277	21,700	(8,513)	18,674	24,212	32,525	14,730	53,115	90,141	34,049
Net earnings (loss)	$ 7,392	$ 17,249	$ 13,368	$ (5,321)	$ 16,825	$ 13,777	$ 19,516	$ 8,838	$ 32,688	$ 58,956	$ 18,057
Net earnings (loss) per share
Basic	$ 0.48	$ 1.11	$ 0.86	$ (0.34)	$ 1.09	$ 0.89	$ 1.26	$ 0.57
Diluted	$ 0.47	$ 1.10	$ 0.85	$ (0.34)	$ 1.07	$ 0.88	$ 1.25	$ 0.57