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								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
Calculation Of Numerator And Denominator In Earnings Per Share [Line Items]
Net income - basic (numerator):	$ 155,661	$ 178,186	$ 201,461	$ 185,169	$ 188,490	$ 256,854	$ 266,586	$ 250,612	$ 720,477	$ 962,542	$ 955,457
Effect of dilutive potential shares
Convertible debentures											56
Net income including conversions - diluted (numerator):									$ 720,477	$ 962,542	$ 955,513
Weighted-average shares - basic (denominator):									139,029	139,027	139,026
Effect of dilutive potential shares
Convertible debentures											21
Stock options and stock appreciation rights									19	11	23
Total weighted-average shares outstanding									139,048	139,038	139,070
Earnings per share:
Basic	$ 1.12	$ 1.28	$ 1.45	$ 1.33	$ 1.36	$ 1.85	$ 1.92	$ 1.80	$ 5.18	$ 6.92	$ 6.87
Diluted	$ 1.12	$ 1.28	$ 1.45	$ 1.33	$ 1.36	$ 1.85	$ 1.92	$ 1.80	$ 5.18	$ 6.92	$ 6.87