Stockholders' Equity and EPS - Reconciliation of Basic EPS to Diluted EPS (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income available to common stockholders	$ 274,817	$ 148,620	$ 707,576	$ 373,296
Denominator:
Weighted average common shares outstanding-basic (in shares)	53,235	52,705	53,062	52,530
Weighted average common shares outstanding—diluted (in shares)	53,919	53,305	53,800	53,230
Earnings per common share:
Basic (dollars per share)	$ 5.16	$ 2.82	$ 13.33	$ 7.11
Diluted (dollars per share)	$ 5.10	$ 2.79	$ 13.15	$ 7.01
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities (in shares)	383	343	404	381
Restricted stock units
Denominator:
Weighted average effect of dilutive securities (in shares)	301	257	334	319