Results of Operations By Quarter (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
Quarterly Results Of Operations [Line Items]
Operating revenue	$ 254,871	$ 227,397	$ 226,561	$ 244,957	$ 223,616	$ 214,594	$ 235,734	$ 200,039	$ 951,489	$ 869,921	$ 665,288
Income from operations	78,397	44,485	56,156	84,927	73,399	80,694	92,155	64,752	263,965	311,000	165,154
Net income	$ 44,246	$ 23,659	$ 30,547	$ 50,974	$ 45,523	$ 51,821	$ 57,196	$ 39,792	$ 149,426	$ 194,332	$ 96,429
Basic earnings per share	$ 0.89	$ 0.48	$ 0.62	$ 1.03	$ 0.93	$ 1.06	$ 1.17	$ 0.81	$ 3.03	$ 3.97	$ 1.99
Diluted earnings per share	$ 0.89	$ 0.48	$ 0.62	$ 1.03	$ 0.93	$ 1.06	$ 1.17	$ 0.81	$ 3.03	$ 3.97	$ 1.99