Stockholders' Equity and EPS Reconciliation of Basic EPS to Diluted EPS (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerator:
Net income available to common stockholders	$ 58,757	$ 67,621	$ 48,584	$ 40,891	$ 50,421	$ 42,289	$ 47,603	$ 34,790	$ 215,853	$ 175,103	$ 171,902
Denominator:
Weighted average common shares outstanding-basic									45,309	44,242	43,004
Denominator for diluted calculation									45,944	44,764	43,637
Earnings per common share:
Basic	$ 1.29	$ 1.48	$ 1.08	$ 0.91	$ 1.13	$ 0.95	$ 1.08	$ 0.79	$ 4.76	$ 3.96	$ 4.00
Diluted	$ 1.27	$ 1.46	$ 1.06	$ 0.90	$ 1.12	$ 0.94	$ 1.06	$ 0.78	$ 4.70	$ 3.91	$ 3.94
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities									431	370	517
Restricted stock units
Denominator:
Weighted average effect of dilutive securities									204	152	116