Earnings Per Share (Summary of Earnings Per Share, Basic and Diluted) (Detail) (USD $) In Thousands, except 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 And Diluted By Common Class [Line Items]
Net income	$ 60,071	$ 59,648	$ 54,401	$ 42,079	$ 37,771	$ 40,514	$ 36,715	$ 32,335	$ 216,199	$ 147,335	$ 107,896
Convertible preferred stock accrued dividends											(1,488)
Earnings attributable to common shareholders for basic earnings per share									216,199	147,335	106,408
Weighted-average shares outstanding									82,553	79,477	33,704
Share-based payment awards classified as participating securities									775	1,133	1,730
Denominator for basic earnings per share	83,378	84,002	83,294	82,565	81,512	80,819	80,151	79,937	83,328	80,610	35,434
Dilutive securities									2,408	3,044	3,055
Convertible preferred stock											42,262
Denominator for diluted earnings per share	85,750	86,224	85,737	85,164	84,035	83,649	83,548	83,378	85,736	83,654	80,751
Basic earnings per share	$ 0.72	$ 0.71	$ 0.65	$ 0.51	$ 0.46	$ 0.50	$ 0.46	$ 0.40	$ 2.59	$ 1.83	$ 3.00
Diluted earnings per share	$ 0.70	$ 0.69	$ 0.63	$ 0.49	$ 0.45	$ 0.48	$ 0.44	$ 0.39	$ 2.52	$ 1.76	$ 1.34
Earnings Per Share, Basic [Member]
Earnings Per Share Basic And Diluted By Common Class [Line Items]
Net income									216,199	147,335	107,896
Convertible preferred stock accrued dividends											(1,488)
Earnings attributable to common shareholders for basic earnings per share									216,199	147,335	106,408
Earnings Per Share, Diluted [Member]
Earnings Per Share Basic And Diluted By Common Class [Line Items]
Earnings attributable to common shareholders for basic earnings per share									216,199	147,335	106,408
Effect of convertible preferred stock											1,488
Net earnings for diluted earnings per share									$ 216,199	$ 147,335	$ 107,896