Fair Value of Assets and Liabilities (Summary of Carrying Amounts and Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest bearing deposits with banks	$ 607,782	$ 998,437
Available-for-sale securities, at fair value	1,716,388	1,792,078
Held-to-maturity securities	884,826	0
Trading account securities	448	1,206
Federal Home Loan Bank and Federal Reserve Bank stock, at cost	101,581	91,582
Brokerage customer receivables	27,631	24,221
Mortgage loans held-for-sale	388,038	351,290
Mortgage servicing rights	9,092	8,435	$ 8,946	$ 6,750
Derivative assets	42,026	35,070
FDIC indemnification asset	0	11,846	$ 85,672
Accrued interest receivable and other	604,917	501,456
Federal Home Loan Bank advances	859,876	733,050
Other borrowings	266,019	196,465
Subordinated notes	140,000	140,000		$ 75,000
Junior subordinated debentures	268,566	249,493
Derivative liabilities	42,458	36,921
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	275,795	230,707
Interest bearing deposits with banks	607,782	998,437
Available-for-sale securities, at fair value	1,716,388	1,792,078
Held-to-maturity securities	884,826	0
Trading account securities	448	1,206
Federal Home Loan Bank and Federal Reserve Bank stock, at cost	101,581	91,582
Brokerage customer receivables	27,631	24,221
Mortgage loans held-for-sale	388,038	351,290
Total Loans	17,266,790	14,636,107
Mortgage servicing rights	9,092	8,435
Nonqualified deferred compensation assets	8,517	7,951
Derivative assets	51,298	47,964
FDIC indemnification asset	0	11,846
Accrued interest receivable and other	193,092	169,156
Total financial assets	21,531,278	18,370,980
Non-maturity deposits	14,634,957	12,142,034
Deposits with stated maturities	4,004,677	4,139,810
Federal Home Loan Bank advances	859,876	733,050
Other borrowings	266,019	196,465
Subordinated notes	140,000	140,000
Junior subordinated debentures	268,566	249,493
Derivative liabilities	45,019	41,180
Accrued interest payable	7,394	8,001
Total financial liabilities	20,226,508	17,650,033
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	275,795	230,707
Interest bearing deposits with banks	607,782	998,437
Available-for-sale securities, at fair value	1,716,388	1,792,078
Held-to-maturity securities	878,111	0
Trading account securities	448	1,206
Federal Home Loan Bank and Federal Reserve Bank stock, at cost	101,581	91,582
Brokerage customer receivables	27,631	24,221
Mortgage loans held-for-sale	388,038	351,290
Total Loans	18,106,829	15,346,266
Mortgage servicing rights	9,092	8,435
Nonqualified deferred compensation assets	8,517	7,951
Derivative assets	51,298	47,964
FDIC indemnification asset	0	11,846
Accrued interest receivable and other	193,092	169,156
Total financial assets	22,364,602	19,081,139
Non-maturity deposits	14,634,957	12,142,034
Deposits with stated maturities	3,998,180	4,143,161
Federal Home Loan Bank advances	863,437	738,113
Other borrowings	266,019	197,883
Subordinated notes	140,302	143,639
Junior subordinated debentures	268,046	250,305
Derivative liabilities	45,019	41,180
Accrued interest payable	7,394	8,001
Total financial liabilities	$ 20,223,354	$ 17,664,316