Long-Term Debt - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified			1 Months Ended	3 Months Ended		12 Months Ended	1 Months Ended	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2010 Trust Preferred Securities	Mar. 31, 2012 Trust Preferred Securities	Mar. 31, 2011 Trust Preferred Securities	Dec. 31, 2006 Trust Preferred Securities	Jul. 31, 2009 Subordinated Notes	Mar. 31, 2012 Subordinated Notes	Mar. 31, 2011 Subordinated Notes	Dec. 31, 2007 Subordinated Notes	Mar. 31, 2012 Long-term Debt Maximum	Dec. 31, 2011 Long-term Debt Maximum
Debt Instrument [Line Items]
Debt instrument face amount						$ 100,000				$ 50,000
Debt instrument quarterly floating coupon rate basis						Three-month LIBOR plus 2.25%				Three-month LIBOR plus 2.25%
Debt instrument quarterly floating coupon rate						2.25%				2.25%
Debt instrument due date						Jan. 30, 2037				Jul. 30, 2037
Interest expense yield				2.86%	2.60%			2.86%	2.60%
Percentage of yield of debt securities				6.88%	6.85%			6.88%	6.85%
Earliest optional redemption date without penalty				Jan. 30, 2012				Jul. 30, 2012
Principal amount of debt repurchased			500				10,000
Accrued interest payable balance on long-term debt	$ 7,774	$ 8,134									$ 1,000	$ 1,000