Reconciliation of Basic EPS to Diluted EPS (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Numerator:
Net income available to common stockholders	$ 50,421	$ 42,289	$ 47,603	$ 34,790	$ 35,574	$ 37,571	$ 65,750	$ 33,007	$ 175,103	$ 171,902	$ 94,951
Denominator:
Weighted average common shares outstanding-basic									44,242	43,004	41,774
Denominator for diluted calculation									44,764	43,637	42,478
Earnings per common share:
Basic	$ 1.13	$ 0.95	$ 1.08	$ 0.79	$ 0.82	$ 0.87	$ 1.53	$ 0.78	$ 3.96	$ 4.00	$ 2.27
Diluted	$ 1.12	$ 0.94	$ 1.06	$ 0.78	$ 0.81	$ 0.86	$ 1.50	$ 0.76	$ 3.91	$ 3.94	$ 2.24
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities									370	517	641
Restricted stock units
Denominator:
Weighted average effect of dilutive securities									152	116	63