Business Combinations (Details) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Oct. 02, 2014	Feb. 14, 2015	Feb. 13, 2015	Jan. 01, 2014	Oct. 01, 2014
Business Acquisition [Line Items]
Certain Loans Acquired in Transfer Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value					$ 149,294,000	$ 316,327,000
Gain on acquisition	37,555,000	0
Merger-related expenses	2,997,000	2,453,000
Bancorporation
Business Acquisition [Line Items]
BancShares number of shares owned in Bancorporation			32,042
Fair value of Bancorporation shares owned by BancShares							29,551,000
Investment securities available for sale							2,011,263,000
Loans							4,491,067,000
Intangible asset							109,416,000
Total assets acquired							8,275,367,000
Deposits							7,174,817,000
Total liabilities assumed							7,655,408,000
Cash paid to shareholders			(30,394,000)
Cash and overnight investments							1,280,000,000
Goodwill acquired			4,233,000
Pro forma revenue	323,524,000
Pro forma net income	37,230,000
Capitol City Bank and Trust
Business Acquisition [Line Items]
Cash and cash equivalents					19,622,000
Investment securities available for sale					35,413,000
Intangible asset					500,000
Other assets					1,731,000
Total assets acquired					206,560,000
Deposits					266,352,000
Short-term borrowings					5,501,000
Other liabilities					667,000
Total liabilities assumed					272,520,000
Fair value of net assets (liabilities) assumed					(65,960,000)
Cash received from FDIC					103,515,000
Gain on acquisition				37,555,000
Merger-related expenses	200,000
Revenue generated	1,300,000
Class A Common Stock | Bancorporation
Business Acquisition [Line Items]
Right to receive number of shares conversion			2,586,762
Value of shares of BancShares common stock issued to Bancorporation							560,370,000
Class B Common Stock | Bancorporation
Business Acquisition [Line Items]
Right to receive number of shares conversion			18,202
Value of shares of BancShares common stock issued to Bancorporation							3,877,000
Shares and Cash | Bancorporation
Business Acquisition [Line Items]
Right to receive cash per share			$ 50.00
Shares and Cash | Class A Common Stock | Bancorporation
Business Acquisition [Line Items]
Right to receive number of shares conversion			4.00
Shares Only | Class A Common Stock | Bancorporation
Business Acquisition [Line Items]
Right to receive number of shares conversion			3.58
Shares Only | Class B Common Stock | Bancorporation
Business Acquisition [Line Items]
Right to receive number of shares conversion			0.42