Fair Value Measurement (Fair Value Disclosure by Balance Sheet Grouping ) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Financial assets:
Cash and Cash Equivalents, at Carrying Value	$ 532,425,000	$ 959,285,000	$ 697,424,000	$ 917,822,000
Cash and due from banks, Fair Value	532,425,000	959,285,000	697,424,000
Available-for-sale Securities, Carrying Amount	3,791,059,000	3,433,171,000	3,545,288,000
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	2,855,174,000	3,079,620,000	2,903,609,000
Held-to-maturity Securities, Carrying Amount		3,079,620,000	2,903,609,000
Held-to-maturity Securities, Fair Value	2,848,912,000	2,990,161,000	2,875,920,000
Other Investments, Carrying Amount	148,475,000	148,446,000	148,654,000
Other Investments, Fair Value	148,475,000	148,446,000	148,654,000
Loans Receivable Held-for-sale, Carrying Amount	3,568,000	7,143,000	13,428,000
Loans Held-for-sale, Fair Value Disclosure	3,568,000	7,143,000	13,428,000
Loans and Leases Receivable, Net Amount	15,344,337,000	14,463,553,000	15,182,498,000
Originated loans and leases receivable, carrying value		10,734,797,000	12,398,116,000
Net noncovered loans, Fair Value	12,588,147,000	10,621,047,000	12,235,530,000
Net covered loans and loss share receivable, Carrying Amount		495,815,000
Net covered loans and loss share receivable, Fair Value	268,459,000	495,815,000	312,659,000
Accrued interest receivable, Carrying Amount	69,086,000	64,555,000	63,657,000
Accrued interest receivable, Fair Value	69,086,000	64,555,000	63,657,000
Derivative Assets, Carrying Amount	72,573,000	46,467,000	55,197,000
Derivative Asset, Fair Value, Gross Asset	72,573,000	46,467,000	55,197,000
Financial liabilities:
Deposits, Carrying Amount	19,925,595,000	19,811,674,000	19,504,665,000
Deposits, Fair Value	19,932,571,000	19,810,963,000	19,510,192,000
Total Federal funds purchased and securities sold under agreements to repurchase, Carrying Amount	1,113,371,000	926,195,000	1,272,591,000
Federal funds purchased and securities sold under agreements to repurchase, Fair Value	1,113,371,000	926,195,000	1,272,591,000
Wholesale Borrowings, Carrying Amount	316,628,000	349,277,000	428,071,000
Other Borrowings, Fair Value	320,180,000	352,923,000	430,676,000
Long-term Debt	512,625,000	324,430,000	505,192,000
Long-term Debt, Fair Value	527,018,000	330,337,000	516,476,000
Accrued interest payable, Carrying Amount	9,620,000	5,682,000	9,820,000
Accrued interest payable, Fair Value	9,620,000	5,682,000	9,820,000
Derivative liabilities, Carrying Amount	65,447,000	55,106,000	55,439,000
Derivative Liability, Fair Value, Gross Liability	65,447,000	55,106,000	55,439,000
Mortgage Servicing rights Valuation Allowance	(1,131,000)	(425,000)	(955,000)	(282,000)
Mortgage Servicing Rights Net Carrying balance	20,359,000	22,044,000	21,100,000
Mortgage Servicing Rights (MSR) Impairment (Recovery)	(176,000)	143,000	(700,000)
Impaired Collateral Dependant Loans, Recorded Investment	75,600,000	65,300,000	60,300,000
Determination Of Specific Allowance For Loan Losses	(11,800,000)	(7,000,000)	(4,300,000)
Fair Value, Inputs, Level 1
Financial assets:
Cash and due from banks, Fair Value		959,285,000	697,424,000
Available-for-sale Securities, Carrying Amount	2,869,000	3,055,000	2,974,000
Held-to-maturity Securities, Fair Value	0	0	0
Other Investments, Fair Value	0	0	0
Loans Held-for-sale, Fair Value Disclosure	0	0	0
Net noncovered loans, Fair Value	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	3,441,959,000	3,080,691,000	3,203,127,000
Held-to-maturity Securities, Fair Value		2,990,161,000	2,875,920,000
Other Investments, Fair Value		148,446,000	148,654,000
Loans Held-for-sale, Fair Value Disclosure	3,568,000	7,143,000	13,428,000
Net noncovered loans, Fair Value	0	0	0
Net covered loans and loss share receivable, Fair Value	0	0	0
Accrued interest receivable, Fair Value	69,086,000	64,555,000	63,657,000
Derivative Asset, Fair Value, Gross Asset	72,573,000	46,467,000	55,197,000
Financial liabilities:
Deposits, Fair Value	19,932,571,000	19,810,963,000	19,510,192,000
Federal funds purchased and securities sold under agreements to repurchase, Fair Value		926,195,000	1,272,591,000
Other Borrowings, Fair Value	320,180,000	352,923,000	430,676,000
Long-term Debt, Fair Value	527,018,000	330,337,000	516,476,000
Accrued interest payable, Fair Value	9,620,000	5,682,000	9,820,000
Derivative Liability, Fair Value, Gross Liability	65,447,000	55,106,000	55,439,000
Level 3
Financial assets:
Cash and due from banks, Fair Value	0	0	0
Available-for-sale Securities, Carrying Amount	346,231,000	349,425,000	339,187,000
Held-to-maturity Securities, Fair Value	0	0	0
Other Investments, Fair Value	0	0	0
Loans Held-for-sale, Fair Value Disclosure	0	0	0
Loans and Leases Receivable, Net Amount		3,367,024,000
Net noncovered loans, Fair Value	12,588,147,000	10,621,047,000	12,235,530,000
Net covered loans and loss share receivable, Fair Value	268,459,000	495,815,000	312,659,000
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	12,758,492,000	10,734,797,000	12,398,116,000
Acquired Loans
Financial assets:
Loans and Leases Receivable, Net Amount	2,317,386,000	3,232,941,000	2,471,723,000
Net noncovered loans, Fair Value	2,393,942,000	3,367,024,000	2,564,842,000
Acquired Loans | Fair Value, Inputs, Level 1
Financial assets:
Net noncovered loans, Fair Value	0	0	0
Acquired Loans | Fair Value, Inputs, Level 2
Financial assets:
Net noncovered loans, Fair Value	0	0	0
Acquired Loans | Level 3
Financial assets:
Net noncovered loans, Fair Value	$ 2,393,942,000		$ 2,564,842,000