Business Combinations Businss Combinations (Details) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 02, 2014	Jan. 02, 2014	Oct. 01, 2014
Business Acquisition [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value						$ 4,175,586,000
Short-term borrowings						296,100,000
Merger-related expenses	13,064,000	391,000	791,000
Bancorporation
Business Acquisition [Line Items]
BancShares number of shares owned in Bancorporation				32,042
Gain on Bancorporation stock				29,129
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	8,000,000
Pro forma revenue	1,336,340,000	1,412,226,000
Pro forma net income	(13,171,000)	210,529,000
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
Loans held for sale					1,183,000
Restricted equity securities					3,776,000
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value					307,927,000
Premises and equipment					2,686,000
Other real estate owned					11,591,000
Other intangible assets					3,780,000
Other assets					16,346,000
Total assets acquired					612,921,000
Deposits					631,871,000
Short-term borrowings					406,000
Other liabilities					3,559,000
Total liabilities assumed					635,836,000
Fair value of net assets (liabilities) acquired					22,915,000
Goodwill acquired	32,915,000	0			32,915,000
Revenue of acquired business	15,200,000
Merger-related expenses	5,000,000
Subsequent adjustment to goodwill for revised fair value estimate in acquisition	(8,400,000)
Shares and Cash
Business Acquisition [Line Items]
Right to receive cash per share	50.00
Shares and Cash | Class A Common Stock
Business Acquisition [Line Items]
Right to receive number of shares conversion	4.00
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 | Class A Common Stock
Business Acquisition [Line Items]
Right to receive number of shares conversion	3.58
Shares Only | Class B Common Stock
Business Acquisition [Line Items]
Right to receive number of shares conversion	0.42
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