Calculation of Net Income 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
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 58,619	$ 57,660	$ 45,329	[1]	$ 48,334	$ 42,624	$ 44,168	$ 36,425	$ 26,705	$ 209,942	$ 149,922	$ 66,069
Net income available to common stockholders-Basic										208,861	147,763	63,729
Basic weighted average shares										138,201	140,479	142,833
Basic net income per share	$ 0.43	$ 0.42	$ 0.33		$ 0.34	$ 0.30	$ 0.31	$ 0.25	$ 0.18	$ 1.51	$ 1.05	$ 0.45
Net income	58,619	57,660	45,329	[1]	48,334	42,624	44,168	36,425	26,705	209,942	149,922	66,069
Net income available to common stockholders-diluted	58,322	57,383	45,101		48,070	42,037	43,556	35,890	26,293	208,867	147,772	63,729
Basic weighted average shares										138,201	140,479	142,833
Dilutive effect of potential common shares										1,208	1,311	1,195
Diluted weighted average shares										139,409	141,790	144,028
Diluted net income per share	$ 0.42	$ 0.41	$ 0.32	[1]	$ 0.34	$ 0.30	$ 0.31	$ 0.25	$ 0.18	$ 1.50	$ 1.04	$ 0.44
Basic
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income allocated to participating securities										1,081	2,159	2,340
Diluted
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income allocated to participating securities										$ 1,075	$ 2,150	$ 2,340

[1] The decrease in second quarter 2012 income before income taxes, net income, and net income per share was primarily due to a charge related to a litigation settlement disclosed in the Company's July 5, 2012, Form 8-K. The settlement resulted in a non-recurring $19.0 million pre-tax charge to selling, general and administrative expenses and reduced second quarter net income and net income per share by $11.4 million and $0.08 per share, respectively.