Business Combinations Business Combinations (Details) - USD ($)			3 Months Ended		9 Months Ended
	Feb. 14, 2015	Oct. 02, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Feb. 13, 2015	Oct. 01, 2014	Jan. 01, 2014
Business Acquisition [Line Items]
Goodwill Impairment Prior To Acquisition		$ 166,800,000
Certain Loans Acquired in Transfer Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value							$ 154,496,000		$ 316,327,000
Gain on acquisition			$ 0	$ 0	$ 42,930,000	$ 0
Merger-related expenses			3,679,000	$ 1,505,000	11,249,000	$ 7,352,000
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
Cash and overnight investments								1,280,000,000
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
Goodwill acquired		$ 4,233,000
Pro forma revenue			341,927,000		995,704,000
Pro forma net income			(127,768,000)		(50,279,000)
Capitol City Bank and Trust
Business Acquisition [Line Items]
Cash and cash equivalents							19,622,000
Investment securities available for sale							35,413,000
Certain Loans Acquired in Transfer Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value							154,496,000
Intangible asset							690,000
Other assets							1,714,000
Total assets acquired							211,935,000
Deposits							266,352,000
Business Combination Bargain Purchase Gain Adjustment			5,400,000
Short-term borrowings							5,501,000
Other liabilities							667,000
Total liabilities assumed							272,520,000
Fair value of net assets (liabilities) assumed							(60,585,000)
Cash received from FDIC							$ 103,515,000
Gain on acquisition	$ 42,930,000
Bargain Purchase After Tax Gain	$ 26,400,000
Merger-related expenses			500,000		1,800,000
Revenue generated			$ 2,300,000		$ 6,000,000
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