Computation of Basic and Diluted Earnings per Share (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 for earnings per share:
Net income	$ 14,999		$ 14,065		$ 13,321		$ 12,099		$ 12,384		$ 13,950		$ 8,277		$ 9,742		$ 54,484		$ 44,353		$ 28,308
Numerator for basic earnings per share																	51,143		11,765		14,310
Earnings per share
Excluded Options due to Anti-Dilutive																	1,534		572		1,803
Common Stock
Numerator for earnings per share:
Net income																	54,484		44,353		28,308
Accretion of preferred stock																			(17,563)		(1,432)
Net income allocated to participating securities																	(3,341)		(15,025)		(17,984)
Effect on income available from redemption of preferred stock																					5,418
Numerator for basic earnings per share																	51,143		11,765		14,310
Effect on income available from reallocation of options																	261		1,185		996
Numerator for diluted earnings per share																	51,404		12,950		15,306
Denominator for basic (common) earnings per share:
Weighted average shares outstanding																	40,190		17,094		17,056
Denominator for diluted earnings per share																	43,821		20,473		19,314
Earnings per share
Basic	$ 0.35	[1]	$ 0.33	[1]	$ 0.31	[1]	$ 0.30	[1]	$ 0.32	[1]	$ 0.36	[1]	$ (0.41)	[1]	$ 0.19	[1]	$ 1.27	[1]	$ 0.69	[1]	$ 0.84
Diluted	$ 0.32	[1]	$ 0.30	[1]	$ 0.29	[1]	$ 0.27	[1]	$ 0.29	[1]	$ 0.33	[1]	$ (0.41)	[1]	$ 0.18	[1]	$ 1.17	[1]	$ 0.63	[1]	$ 0.79
Common Stock | Stock Options
Denominator for basic (common) earnings per share:
Effect of dilutive securities																	3,631		3,379		2,258
Puttable common stock
Numerator for earnings per share:
Numerator for basic earnings per share																			26		118
Effect on income available from reallocation of options																			(12)		(7)
Numerator for diluted earnings per share																			$ 14		$ 111
Denominator for basic and diluted earnings per share:
Weighted average puttable common shares outstanding																			18		141
Earnings per share
Basic																			$ 1.42		$ 0.84
Diluted																			$ 0.77		$ 0.79

[1] Earnings per share amounts for each quarter may not necessarily total to the yearly earnings per share due to the weighting of shares outstanding on a quarterly and year-to-date basis.