Borrowed Funds (Narrative) (Details) (USD $)	3 Months Ended	12 Months Ended					1 Months Ended		1 Months Ended		1 Months Ended
	Mar. 31, 2013	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2012		Mar. 31, 2012	Oct. 31, 2002 SFG Capital Trust I [Member]	Mar. 31, 2013 SFG Capital Trust I [Member]	Mar. 31, 2004 SFG Capital Trust II [Member]	Mar. 31, 2013 SFG Capital Trust II [Member]	Dec. 31, 2005 SFG Capital Trust III [Member]	Mar. 31, 2013 SFG Capital Trust III [Member]	Mar. 31, 2013 Subordinated Debentures Owed To Unconsolidated Subsidiary Trusts [Member]	Dec. 31, 2012 Subordinated Debentures Owed To Unconsolidated Subsidiary Trusts [Member]	Mar. 31, 2012 Subordinated Debentures Owed To Unconsolidated Subsidiary Trusts [Member]	Dec. 31, 2009 Director Of Summit [Member]	Dec. 31, 2009 Unrelated Party One [Member]	Dec. 31, 2009 Unrelated Party Two [Member]
Debt Instrument [Line Items]
Long-term borrowings	$ 163,588,000			$ 203,268,000		$ 267,121,000							$ 19,600,000	$ 19,600,000	$ 19,600,000
Portion of term loans with a variable rate of interest	5,400,000
Variable interest rate, basis points	0.50%		2.75%					3.45%		2.80%		1.45%
Portion of term loans with a fixed rate of interest	3,500,000
Term loan, fixed interest rate	8.00%
Term loan maturity date	Jan. 01, 2017
Average interest rate paid on long-term borrowings	3.82%					3.97%
Subordinated debentures	16,800,000	6,800,000	10,000,000	16,800,000	[1]	16,800,000										5,000,000	1,000,000	800,000
Time until debt matures, threshold to qualify as Tier 2 capital, years	5 years
Amount qualifying as Tier 2 capital reduced by percent each year until maturity	20.00%
Subordinated debentures, interest rate		10.00%
Subordinated borrowing, term		10 years	7 years 6 months
Subordinated borrowing, not pre-payable within number of years		5 years
Percentage of common equity owned of Capital Trust I, Capital Trust II and Capital Trust III	100.00%
Issued capital securities							3,500,000		7,500,000		8,000,000
Issued common securities							109,000		232,000		248,000
Investment in debentures							$ 3,610,000		$ 7,730,000		$ 8,250,000
Trust preferred securities limited to Tier 1 capital elements, net of goodwill	25.00%

[1] December 31, 2012 financial information has been extracted from audited consolidated financial statements