Business Combinations Businss Combinations (Details) - USD ($)				12 Months Ended
	Feb. 14, 2015	Oct. 02, 2014	Jan. 02, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Feb. 13, 2015	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				$ 154,496,000	$ 623,408,000		$ 154,496,000
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value								$ 4,175,586,000
FDIC Indemnification Asset, Acquisitions				0	5,106,000	$ 0
Gain on acquisition				42,930,000	0	0
Merger-related expenses				14,174,000	13,064,000	$ 391,000
Capitol City Bank and Trust
Business Acquisition [Line Items]
Cash and due from banks							19,622,000
Investment securities							35,413,000
Other intangible assets							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
Gain on acquisition	$ 42,930,000
Bargain Purchase After Tax Gain	$ 26,400,000
Short-term borrowings							5,501,000
Other liabilities							667,000
Total liabilities assumed							272,520,000
Fair value of net assets (liabilities) acquired							(60,585,000)
Business Combination Cash Received From FDIC In Acquisition							$ 103,515,000
Revenue of acquired business				8,300,000
Merger-related expenses				1,900,000
Bancorporation
Business Acquisition [Line Items]
BancShares number of shares owned in Bancorporation		32,042
Gain on Bancorporation stock		$ 29,100,000
Cash paid to shareholders		30,394,000
Fair value of Bancorporation shares owned by BancShares								29,551,000
Total purchase price		624,192,000
Cash and due from banks								194,570,000
Overnight investments								1,087,325,000
Investment securities								2,011,263,000
Loans held for sale								30,997,000
Loans and leases								4,491,067,000
Premises and equipment								238,646,000
Other real estate owned								35,344,000
Income earned not collected								15,266,000
FDIC loss share receivable								5,106,000
Other intangible assets								109,416,000
Other assets								56,367,000
Total assets acquired								8,275,367,000
Deposits								7,174,817,000
Short-term borrowings								295,681,000
Long-term obligations								124,852,000
FDIC loss share payable								224,000
Other liabilities								59,834,000
Total liabilities assumed								7,655,408,000
Fair value of net assets (liabilities) acquired								619,959,000
Goodwill acquired		4,233,000		0	4,233,000
Revenue of acquired business				92,800,000
Net income of acquired business				12,700,000
Merger-related expenses				12,300,000	8,000,000
Pro forma revenue				1,336,340,000
Pro forma net income				(13,171,000)
Goodwill Impairment Prior To Acquisition		$ 166,800,000
Bancorporation | Class A Common Stock
Business Acquisition [Line Items]
Right to receive number of shares conversion		2,586,762
Value of shares of BancShares common stock issued to Bancorporation shareholders								560,370,000
Bancorporation | Class B Common Stock
Business Acquisition [Line Items]
Right to receive number of shares conversion		18,202
Value of shares of BancShares common stock issued to Bancorporation shareholders								$ 3,877,000
1st Financial
Business Acquisition [Line Items]
Cash paid to shareholders			$ 2,000,000
Cash paid to acquire and retire TARP securities			8,000,000
Total purchase price			10,000,000
Cash and due from banks			28,194,000
Investment securities			237,438,000
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value			307,927,000
Other intangible assets			3,780,000
Total assets acquired			612,921,000
Deposits			631,871,000
Total liabilities assumed			635,836,000
Goodwill acquired			$ 32,915,000	0	$ 32,915,000
Revenue of acquired business				15,200,000
Merger-related expenses				$ 5,000,000
Shares and Cash | Bancorporation
Business Acquisition [Line Items]
Right to receive cash per share		$ 50.00
Shares and Cash | Bancorporation | Class A Common Stock
Business Acquisition [Line Items]
Right to receive number of shares conversion		4.00
Shares Only | Bancorporation | Class A Common Stock
Business Acquisition [Line Items]
Right to receive number of shares conversion		3.58
Shares Only | Bancorporation | Class B Common Stock
Business Acquisition [Line Items]
Right to receive number of shares conversion		0.42