Quarterly Financial Information (Detail) - 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
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 338,566	$ 422,251	$ 472,761	$ 657,699	$ 901,219	$ 845,628	$ 757,276	$ 678,168	$ 1,891,277	$ 3,182,291	$ 2,716,034
Operating income (loss)	(76,796)	(329,515)	(24,764)	24,103	106,833	30,291	87,226	58,776	(406,972)	283,126	322,191
Net income (loss)	$ (58,658)	$ (225,978)	$ (18,975)	$ 9,125	$ 57,583	$ 15,976	$ 54,283	$ 34,822	$ (294,486)	$ 162,664	$ 188,009
Net income (loss) per common share:
Basic	$ (0.40)	$ (1.54)	$ (0.13)	$ 0.06	$ 0.39	$ 0.11	$ 0.37	$ 0.24	$ (2.00)	$ 1.12	$ 1.29
Diluted	$ (0.40)	$ (1.54)	$ (0.13)	$ 0.06	$ 0.39	$ 0.11	$ 0.37	$ 0.24	$ (2.00)	$ 1.11	$ 1.28