Fair Value Measurement (Fair Value Disclosure by Balance Sheet Grouping ) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2013
Financial assets:
Cash and Cash Equivalents, at Carrying Value	$ 411,587	$ 485,328	$ 411,587	$ 485,328	$ 697,424			$ 917,822
Cash and due from banks, Fair Value	411,587	485,328	411,587	485,328	697,424
Available-for-sale Securities, Carrying Amount	3,925,724	3,462,990	3,925,724	3,462,990	3,545,288
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	2,728,638	3,002,262	2,728,638	3,002,262	2,903,609
Held-to-maturity Securities, Carrying Amount		3,002,262		3,002,262	2,903,609
Held-to-maturity Securities, Fair Value	2,728,911	2,947,172	2,728,911	2,947,172	2,875,920
Other Investments, Carrying Amount	147,955	148,421	147,955	148,421	148,654
Other Investments, Fair Value	147,955	148,421	147,955	148,421	148,654
Loans Receivable Held-for-sale, Carrying Amount	5,384	19,512	5,384	19,512	13,428
Loans Held-for-sale, Fair Value Disclosure	7,781	19,512	7,781	19,512	14,389
Loans and Leases Receivable, Net Amount	15,645,830	15,101,949	15,645,830	15,101,949	15,182,498
Originated loans and leases receivable, carrying value		11,980,876		11,980,876	12,398,116
Net noncovered loans, Fair Value	13,344,826	11,995,816	13,344,826	11,995,816	12,235,530
Net covered loans and loss share receivable, Carrying Amount		382,175		382,175	312,659
Net covered loans and loss share receivable, Fair Value	190,795	382,175	190,795	382,175	312,659
Accrued interest receivable, Carrying Amount	75,404	65,736	75,404	65,736	63,657
Accrued interest receivable, Fair Value	75,404	65,736	75,404	65,736	63,657
Derivative Assets, Carrying Amount	76,196	43,097	76,196	43,097	55,197
Derivative Asset, Fair Value, Gross Asset	76,196	43,097	76,196	43,097	55,197
Financial liabilities:
Deposits, Carrying Amount	19,821,916	19,366,911	19,821,916	19,366,911	19,504,665
Deposits, Fair Value	19,830,705	19,370,624	19,830,705	19,370,624	19,510,192
Total Federal funds purchased and securities sold under agreements to repurchase, Carrying Amount	1,208,275	1,273,290	1,208,275	1,273,290	1,272,591
Federal funds purchased and securities sold under agreements to repurchase, Fair Value	1,208,275	1,273,290	1,208,275	1,273,290	1,272,591
Wholesale Borrowings, Carrying Amount	381,299	608,463	381,299	608,463	428,071
Other Borrowings, Fair Value	385,216	611,753	385,216	611,753	430,676
Long-term Debt	508,947	249,933	508,947	249,933	505,192
Long-term Debt, Fair Value	514,640	259,051	514,640	259,051	516,476
Accrued interest payable, Carrying Amount	10,062	5,310	10,062	5,310	9,820
Accrued interest payable, Fair Value	10,062	5,310	10,062	5,310	9,820
Derivative liabilities, Carrying Amount	67,936	49,280	67,936	49,280	55,439
Derivative Liability, Fair Value, Gross Liability	67,936	49,280	67,936	49,280	55,439
Mortgage Servicing rights Valuation Allowance	(550)	(505)	(550)	(505)	(955)	$ (490)	$ (562)	$ (282)
Mortgage Servicing Rights Net Carrying balance	19,217	21,684	19,217	21,684	21,100
Mortgage Servicing Rights (MSR) Impairment (Recovery)	(60)	(57)	(405)	223	(700)
Impaired Collateral Dependant Loans, Recorded Investment	89,800	59,900	89,800	59,900	60,300
Determination Of Specific Allowance For Loan Losses	(9,400)	(4,500)	(9,400)	(4,500)	(4,300)
Fair Value, Inputs, Level 1
Financial assets:
Cash and due from banks, Fair Value		485,328		485,328	697,424
Available-for-sale Securities, Carrying Amount	2,781	2,916	2,781	2,916	2,974
Held-to-maturity Securities, Fair Value	0	0	0	0	0
Other Investments, Fair Value	0	0	0	0	0
Loans Held-for-sale, Fair Value Disclosure	0	0	0	0	0
Net noncovered loans, Fair Value	0	0	0	0	0
Net covered loans and loss share receivable, Fair Value	0	0	0	0	0
Accrued interest receivable, Fair Value	0	0	0	0	0
Derivative Asset, Fair Value, Gross Asset	0	0	0	0	0
Financial liabilities:
Deposits, Fair Value	0	0	0	0	0
Federal funds purchased and securities sold under agreements to repurchase, Fair Value	0	0	0	0	0
Other Borrowings, Fair Value	0	0	0	0	0
Long-term Debt, Fair Value	0	0	0	0	0
Accrued interest payable, Fair Value	0	0	0	0	0
Derivative Liability, Fair Value, Gross Liability	0	0	0	0	0
Fair Value, Inputs, Level 2
Financial assets:
Cash and due from banks, Fair Value	0	0	0	0	0
Available-for-sale Securities, Carrying Amount	3,575,950	3,112,553	3,575,950	3,112,553	3,203,127
Held-to-maturity Securities, Fair Value		2,947,172		2,947,172	2,875,920
Other Investments, Fair Value		148,421		148,421	148,654
Loans Held-for-sale, Fair Value Disclosure	7,781	19,512	7,781	19,512	14,389
Net noncovered loans, Fair Value	0	0	0	0	0
Net covered loans and loss share receivable, Fair Value	0	0	0	0	0
Accrued interest receivable, Fair Value	75,404	65,736	75,404	65,736	63,657
Derivative Asset, Fair Value, Gross Asset	76,196	43,097	76,196	43,097	55,197
Financial liabilities:
Deposits, Fair Value	19,830,705	19,370,624	19,830,705	19,370,624	19,510,192
Federal funds purchased and securities sold under agreements to repurchase, Fair Value		1,273,290		1,273,290	1,272,591
Other Borrowings, Fair Value	385,216	611,753	385,216	611,753	430,676
Long-term Debt, Fair Value	514,640	259,051	514,640	259,051	516,476
Accrued interest payable, Fair Value	10,062	5,310	10,062	5,310	9,820
Derivative Liability, Fair Value, Gross Liability	67,936	48,765	67,936	48,765	55,439
Level 3
Financial assets:
Cash and due from banks, Fair Value	0	0	0	0	0
Available-for-sale Securities, Carrying Amount	346,993	347,521	346,993	347,521	339,187
Held-to-maturity Securities, Fair Value	0	0	0	0	0
Other Investments, Fair Value	0	0	0	0	0
Loans Held-for-sale, Fair Value Disclosure	0	0	0	0	0
Loans and Leases Receivable, Net Amount		2,869,303		2,869,303
Net noncovered loans, Fair Value	13,344,826	11,995,816	13,344,826	11,995,816	12,235,530
Net covered loans and loss share receivable, Fair Value	190,795	382,175	190,795	382,175	312,659
Accrued interest receivable, Fair Value	0	0	0	0	0
Derivative Asset, Fair Value, Gross Asset	0	0	0	0	0
Financial liabilities:
Deposits, Fair Value	0	0	0	0	0
Federal funds purchased and securities sold under agreements to repurchase, Fair Value	0	0	0	0	0
Other Borrowings, Fair Value	0	0	0	0	0
Long-term Debt, Fair Value	0	0	0	0	0
Accrued interest payable, Fair Value	0	0	0	0	0
Derivative Liability, Fair Value, Gross Liability	0	515	0	515	0
Originated Loans
Financial assets:
Loans and Leases Receivable, Net Amount	13,544,270	11,980,876	13,544,270	11,980,876	12,398,116
Acquired Loans
Financial assets:
Loans and Leases Receivable, Net Amount	1,910,765	2,738,898	1,910,765	2,738,898	2,471,723
Net noncovered loans, Fair Value	1,979,065	2,869,303	1,979,065	2,869,303	2,564,842
Acquired Loans | Fair Value, Inputs, Level 1
Financial assets:
Net noncovered loans, Fair Value	0	0	0	0	0
Acquired Loans | Fair Value, Inputs, Level 2
Financial assets:
Net noncovered loans, Fair Value	0	$ 0	0	$ 0	0
Acquired Loans | Level 3
Financial assets:
Net noncovered loans, Fair Value	$ 1,979,065		$ 1,979,065		$ 2,564,842