Earnings Per Share - Reconciliation of Numerators and Denominators of Basic and Diluted Per-Share Computations (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]
Net income - basic and diluted numerator:	$ 94,748	$ 178,186	$ 456,071	$ 564,816
Weighted average shares - basic (denominator):	139,035	139,030	139,034	139,029
Effect of dilutive potential shares
Stock options and stock appreciation rights	30	23	38	17
Total weighted-average shares outstanding	139,065	139,053	139,072	139,046
Earnings per share:
Basic	$ 0.68	$ 1.28	$ 3.28	$ 4.06
Diluted	$ 0.68	$ 1.28	$ 3.28	$ 4.06