Results of Operations by Quarter (Detail) (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, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 239,253		$ 256,685	$ 245,877	$ 233,732	$ 254,871	$ 227,397	$ 226,561	$ 244,957	$ 974,179	$ 951,489	$ 869,921
Income from operations	37,206	[1]	62,422	69,525	72,828	78,397	44,485	56,156	84,927	241,981	263,965	311,000
Net income	$ 1,752	[1],[2]	$ 36,102	$ 39,022	$ 40,758	$ 44,246	$ 23,659	$ 30,547	$ 50,974	$ 117,634	$ 149,426	$ 194,332
Basic earnings per share	$ 0.04		$ 0.72	$ 0.78	$ 0.82	$ 0.89	$ 0.48	$ 0.62	$ 1.04	$ 2.36	$ 3.03	$ 3.97
Diluted earnings per share	$ 0.04		$ 0.72	$ 0.78	$ 0.82	$ 0.89	$ 0.48	$ 0.62	$ 1.04	$ 2.36	$ 3.03	$ 3.97

[1] Includes franchise tax settlement of $12,590 before income tax effect ($8,058, net of income tax effect).

[2] Includes loss on early extinguishment of debt of $27,279 before income tax effect ($17,459 net of income tax effect).