Stockholders' Equity and EPS - Reconciliation of Basic EPS to Diluted EPS (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator:
Net income available to common stockholders	$ 92,964	$ 86,143	$ 172,138	$ 174,659
Denominator:
Weighted average common shares outstanding-basic (in shares)	51,831	51,268	51,739	51,139
Denominator for diluted calculation (in shares)	52,187	51,876	52,130	51,788
Earnings per common share:
Basic (dollars per share)	$ 1.79	$ 1.68	$ 3.33	$ 3.42
Diluted (dollars per share)	$ 1.78	$ 1.66	$ 3.30	$ 3.37
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities (in shares)	238	410	246	420
Restricted stock units
Denominator:
Weighted average effect of dilutive securities (in shares)	118	198	145	229