Stockholders' Equity and EPS Reconciliation of Basic EPS to Diluted EPS (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net income available to common stockholders	$ 99,466	$ 111,081	$ 92,964	$ 79,174	$ 87,512	$ 81,733	$ 86,143	$ 88,516	$ 382,685	$ 343,904	$ 263,870
Denominator:
Weighted average common shares outstanding-basic (in shares)									51,915	51,318	48,931
Denominator for diluted calculation (in shares)									52,349	51,916	49,662
Earnings per common share:
Basic (usd per share)	$ 1.91	$ 2.13	$ 1.79	$ 1.53	$ 1.70	$ 1.59	$ 1.68	$ 1.74	$ 7.37	$ 6.70	$ 5.39
Diluted (usd per share)	$ 1.89	$ 2.12	$ 1.78	$ 1.52	$ 1.68	$ 1.57	$ 1.66	$ 1.71	$ 7.31	$ 6.62	$ 5.31
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities (in shares)									254	387	485
Restricted stock units
Denominator:
Weighted average effect of dilutive securities (in shares)									180	211	246