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		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Numerator:
Net income available to common stockholders	$ 67,621	$ 42,289	$ 157,096	$ 124,682
Denominator:
Weighted average common shares outstanding-basic (in shares)	45,580	44,449	45,180	44,147
Denominator for diluted calculation (in shares)	46,202	44,915	45,765	44,692
Earnings per common share:
Basic (dollars per share)	$ 1.48	$ 0.95	$ 3.48	$ 2.82
Diluted (dollars per share)	$ 1.46	$ 0.94	$ 3.43	$ 2.79
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities (in shares)	429	346	405	402
Restricted stock units
Denominator:
Weighted average effect of dilutive securities (in shares)	193	120	180	143