Calculation of net income per common share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 36,554	$ 34,102	$ 23,520
Less: Dividends - preferred stock	0	0	6,149
Net income available to common stockholders	$ 36,554	$ 34,102	$ 17,371
Basic:
Basic weighted average EPS shares	24,573	24,083	22,339
Basic net income per share	$ 1.49	$ 1.42	$ 1.05
Basic net income per common share	$ 1.49	$ 1.42	$ 0.78
Diluted:
Basic weighted average EPS shares	24,573	24,083	22,339
Dilutive common stock equivalents	84	66	41
Dilutive EPS shares	24,657	24,149	22,380
Diluted net income per share	$ 1.48	$ 1.41	$ 1.05
Diluted net income per common share	$ 1.48	$ 1.41	$ 0.78
Anti-dilutive shares	421	621	776