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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Numerator:
Net income available to common stockholders	$ 48,584	$ 47,603	$ 89,475	$ 82,393
Denominator:
Weighted average common shares outstanding-basic (in shares)	45,164	44,207	44,985	43,994
Denominator for diluted calculation (in shares)	45,684	44,712	45,537	44,573
Earnings per common share:
Basic (dollars per share)	$ 1.08	$ 1.08	$ 1.99	$ 1.87
Diluted (dollars per share)	$ 1.06	$ 1.06	$ 1.96	$ 1.85
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities (in shares)	380	385	384	426
Restricted stock units
Denominator:
Weighted average effect of dilutive securities (in shares)	140	120	168	153