Participation in US Treasury Capital Purchase Program (CPP) (Details) (USD $)	3 Months Ended	12 Months Ended			12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended		12 Months Ended
	Jun. 30, 2012	Dec. 31, 2012	Mar. 31, 2013	Dec. 31, 2011	Dec. 31, 2009 Common Stock [Member]	Sep. 30, 2012 Warrants [Member]	Dec. 31, 2012 Warrants [Member]	Jun. 30, 2012 Preferred Stock [Member]	Dec. 31, 2011 Preferred Stock [Member]	Dec. 31, 2008 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock issued (in shares)		0	0							95,000
Preferred stock issued to U.S. treasury										$ 95,000,000
Preferred stock dividend rate (in hundredths)										5.00%
Number of preferred shares redeemed under CPP (in shares)								71,250	23,750
Value of preferred shares redeemed under CPP								71,600,000	23,900,000
Accrued but unpaid dividends	346,000			142,000
Accretion of discount	1,700,000
Warrants issued to purchase common stock (in shares)		534,910
Exercise price of warrants (in dollars per share)		$ 26.64
Expiry period of warrants from the date of issuance		10 years
Sale of common stock in public offering (in shares)					8,855,000
Net proceed from sale of common stock in public offering					109,000,000
Percentage of number of shares reduced issuable upon exercise of warrants (in hundredths)							50.00%
Number of shares reduced issuable upon exercise of warrants (in shares)							267,455
Repurchased outstanding warrants held by treasury						$ 755,000