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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Numerator:
Net income available to common stockholders	$ 62,995	$ 67,621	$ 205,093	$ 157,096
Denominator:
Weighted average common shares outstanding-basic (in shares)	50,752	45,580	48,281	45,180
Denominator for diluted calculation (in shares)	51,571	46,202	49,200	45,765
Earnings per common share:
Basic (dollars per share)	$ 1.24	$ 1.48	$ 4.25	$ 3.48
Diluted (dollars per share)	$ 1.22	$ 1.46	$ 4.17	$ 3.43
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities (in shares)	534	429	580	405
Restricted stock units
Denominator:
Weighted average effect of dilutive securities (in shares)	285	193	339	180