Fair Value Measurement (Fair Value Disclosure by Balance Sheet Grouping ) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Financial assets:
Cash and Cash Equivalents, at Carrying Value	$ 759,897	$ 532,425	$ 463,817	$ 697,424
Cash and due from banks, Fair Value	759,897	532,425	463,817
Available-for-sale Securities, Carrying Amount	4,104,214	3,791,059	3,967,735
Held-to-maturity Securities, Carrying Amount	2,613,700	2,855,174	2,674,093
Held-to-maturity Securities, Fair Value	2,632,274	2,848,912	2,659,119
Other Investments, Carrying Amount	148,159	148,475	148,172
Other Investments, Fair Value	148,159	148,475	148,172
Loans held for sale	5,249	3,568	5,472
Loans and Leases Receivable, Net Amount	16,073,513	15,344,337	15,923,254
Net covered loans and loss share receivable, Fair Value	157,632	268,459	170,690
Accrued interest receivable, Carrying Amount	70,280	69,086	67,887
Accrued interest receivable, Fair Value	70,280	69,086	67,887
Derivative Assets, Carrying Amount	98,549	72,573	58,107
Derivative Asset, Fair Value, Gross Asset	98,549	72,573	58,107
Financial liabilities:
Deposits, Carrying Amount	21,101,366	19,925,595	20,108,003
Deposits, Fair Value	21,109,594	19,932,571	20,116,298
Total Federal funds purchased and securities sold under agreements to repurchase, Carrying Amount	719,850	1,113,371	1,037,075
Federal funds purchased and securities sold under agreements to repurchase, Fair Value	719,850	1,113,371	1,037,075
Wholesale Borrowings, Carrying Amount	378,996	316,628	580,648
Other Borrowings, Fair Value	383,432	320,180	582,120
Long-term Debt	519,249	512,625	505,173
Long-term Debt, Fair Value	504,468	527,018	503,675
Accrued interest payable, Carrying Amount	10,093	9,620	10,758
Accrued interest payable, Fair Value	10,093	9,620	10,758
Derivative liabilities, Carrying Amount	78,621	65,447	52,745
Derivative Liability, Fair Value, Gross Liability	78,621	65,447	52,745
Mortgage Servicing rights Valuation Allowance	(868)	(1,131)	(399)	$ (955)
Mortgage Servicing Rights Net Carrying balance	17,259	20,359	18,500
Mortgage Servicing Rights (MSR) Impairment (Recovery)	469	176	600
Impaired Collateral Dependant Loans, Recorded Investment	94,100	75,600	84,300
Determination Of Specific Allowance For Loan Losses	7,800	11,800	12,900
Fair Value, Inputs, Level 1
Financial assets:
Cash and due from banks, Fair Value		532,425	463,817
Available-for-sale Securities, Carrying Amount	2,873	2,869	2,821
Held-to-maturity Securities, Fair Value	0	0	0
Other Investments, Fair Value	0	0	0
Loans held for sale	0	0	0
Net covered loans and loss share receivable, Fair Value	0	0	0
Accrued interest receivable, Fair Value	0	0	0
Derivative Asset, Fair Value, Gross Asset	0	0	0
Financial liabilities:
Deposits, Fair Value	0	0	0
Federal funds purchased and securities sold under agreements to repurchase, Fair Value	0	0	0
Other Borrowings, Fair Value	0	0	0
Long-term Debt, Fair Value	0	0	0
Accrued interest payable, Fair Value	0	0	0
Derivative Liability, Fair Value, Gross Liability	0	0	0
Fair Value, Inputs, Level 2
Financial assets:
Cash and due from banks, Fair Value	0	0	0
Available-for-sale Securities, Carrying Amount	3,767,184	3,441,959	3,623,270
Held-to-maturity Securities, Fair Value		2,848,912	2,659,119
Other Investments, Fair Value		148,475	148,172
Loans held for sale	5,249	3,568	5,472
Net covered loans and loss share receivable, Fair Value	0	0	0
Accrued interest receivable, Fair Value	70,280	69,086	67,887
Derivative Asset, Fair Value, Gross Asset	98,549	72,573	58,107
Financial liabilities:
Deposits, Fair Value	21,109,594	19,932,571	20,116,298
Federal funds purchased and securities sold under agreements to repurchase, Fair Value		1,113,371	1,037,075
Other Borrowings, Fair Value	383,432	320,180	582,120
Long-term Debt, Fair Value	504,468	527,018	503,675
Accrued interest payable, Fair Value	10,093	9,620	10,758
Derivative Liability, Fair Value, Gross Liability	78,621	65,447	52,745
Level 3
Financial assets:
Cash and due from banks, Fair Value	0	0	0
Available-for-sale Securities, Carrying Amount	334,157	346,231	341,644
Held-to-maturity Securities, Fair Value	0	0	0
Other Investments, Fair Value	0	0	0
Loans held for sale	0	0	0
Net covered loans and loss share receivable, Fair Value		268,459
Accrued interest receivable, Fair Value	0	0	0
Derivative Asset, Fair Value, Gross Asset	0	0	0
Financial liabilities:
Deposits, Fair Value	0	0	0
Federal funds purchased and securities sold under agreements to repurchase, Fair Value	0	0	0
Other Borrowings, Fair Value	0	0	0
Long-term Debt, Fair Value	0	0	0
Accrued interest payable, Fair Value	0	0	0
Derivative Liability, Fair Value, Gross Liability	0	0	0
Originated Loans
Financial assets:
Loans and Leases Receivable, Net Amount	14,286,598	12,758,492	14,013,370
Originated Loans and Leases Receivable Fair Value	14,069,307	12,588,147	13,795,058
Originated Loans | Fair Value, Inputs, Level 1
Financial assets:
Loans and Leases Receivable, Net Amount	0	0	0
Originated Loans | Fair Value, Inputs, Level 2
Financial assets:
Loans and Leases Receivable, Net Amount	0	0	0
Originated Loans | Level 3
Financial assets:
Loans and Leases Receivable, Net Amount	14,069,307	12,588,147	13,795,058
Acquired Loans
Financial assets:
Loans and Leases Receivable, Net Amount	1,629,283	2,317,386	1,739,194
Originated Loans and Leases Receivable Fair Value	1,681,087	2,393,942	1,796,314
Acquired Loans | Fair Value, Inputs, Level 1
Financial assets:
Loans and Leases Receivable, Net Amount	0	0	0
Acquired Loans | Fair Value, Inputs, Level 2
Financial assets:
Loans and Leases Receivable, Net Amount	0	0	0
Acquired Loans | Level 3
Financial assets:
Loans and Leases Receivable, Net Amount	$ 1,681,087	$ 2,393,942	$ 1,796,314