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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income available to common stockholders	$ 117,210	$ 148,620	$ 123,193	$ 101,483	$ 99,466	$ 111,081	$ 92,964	$ 79,174	$ 490,506	[1]	$ 382,685	$ 343,904
Denominator:
Weighted average common shares outstanding-basic (in shares)									52,588		51,915	51,318
Denominator for diluted calculation (in shares)									53,306		52,349	51,916
Earnings per common share:
Basic (usd per share)	$ 2.22	$ 2.82	$ 2.34	$ 1.94	$ 1.91	$ 2.13	$ 1.79	$ 1.53	$ 9.33	[1]	$ 7.37	$ 6.70
Diluted (usd per share)	$ 2.19	$ 2.79	$ 2.32	$ 1.91	$ 1.89	$ 2.12	$ 1.78	$ 1.52	$ 9.20	[1]	$ 7.31	$ 6.62
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities (in shares)									385		254	387
Restricted stock units
Denominator:
Weighted average effect of dilutive securities (in shares)									333		180	211

[1] Included in income tax expense, net income available to common stockholders, earnings per common share-basic and earnings for common share-diluted, for the year ended December 31, 2017, are tax benefits recognized associated with the adoption of Accounting Standards Update ("ASU") 2016-09, Improvements to Employee Share-Based Payment Accounting in the first quarter of 2017. This guidance was adopted on a prospective basis with no change to prior period amounts.