Reconciliation Income (Loss) from Continuing Operations, Income (Loss) from Discontinued Operations and Net Income (Loss) and Weighted Average Common Shares Outstanding for Calculating Basic Net Earnings (Loss) Per Share (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
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Income from continuing operations	$ 54,224	$ 154,111	$ 116,663	$ 49,252	$ 60,918	$ 128,495	$ 74,817	$ 40,699	$ 374,250		$ 304,929		$ 255,072
Income (loss) from discontinued operations, net of tax									(16,180)		4,684		12,755
Net income	55,478	153,812	107,026	41,754	64,848	137,422	81,110	26,233	358,070		309,613		267,827
Less: Distributed earnings to common shareholders									214,541		158,708		150,724
Less: Distributed earnings to participating securities									86		68		51
Undistributed earnings									143,443		150,837		117,052
Undistributed earnings allocated to common shareholders									143,385		150,772		117,014
Undistributed earnings allocated to participating securities									58		65		38
Total income from continuing operations available to common shareholders, basic									374,106		304,796		254,983
Total income available to common shareholders, basic									$ 357,926		$ 309,480		$ 267,738
Weighted-average common shares outstanding, basic									176,445		175,484		174,833
Income from continuing operations	$ 0.31	$ 0.87	$ 0.66	$ 0.28	$ 0.35	$ 0.73	$ 0.43	$ 0.23	$ 2.12	[1]	$ 1.74	[1]	$ 1.46	[1]
Income (loss) from discontinued operations, net of tax									$ (0.09)	[1]	$ 0.03	[1]	$ 0.07	[1]
Net income	$ 0.31	$ 0.87	$ 0.61	$ 0.24	$ 0.37	$ 0.78	$ 0.46	$ 0.15	$ 2.03	[1]	$ 1.76	[1]	$ 1.53	[1]

[1] Amounts may not sum due to rounding.