Fair Value of Assets and Liabilities (Summary of Carrying Amounts and Estimated Fair Values of Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest bearing deposits with banks	$ 998,437	$ 495,574
Available-for-sale securities, at fair value	1,792,078	2,176,290
Trading account securities	1,206	497
Federal Home Loan Bank and Federal Reserve Bank stock, at cost	91,582	79,261
Brokerage customer receivables	24,221	30,953
Mortgage servicing rights	8,435	8,946	6,750	6,700
Derivative assets	35,070	29,130
FDIC indemnification asset	11,846	85,672	208,160
Federal Home Loan Bank advances	733,050	417,762
Other borrowings	196,465	255,104
Subordinated notes	140,000	0	75,000
Junior subordinated debentures	249,493	249,493
Derivative liabilities	36,921	34,807
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash Equivalents	230,707	263,864
Interest bearing deposits with banks	998,437	495,574
Available-for-sale securities, at fair value	1,792,078	2,176,290
Trading account securities	1,206	497
Federal Home Loan Bank and Federal Reserve Bank stock, at cost	91,582	79,261
Brokerage customer receivables	24,221	30,953
Mortgage loans held-for-sale	351,290	332,485
Mortgage loans held-for-sale, at lower of cost or market	0	1,842
Total Loans	14,636,107	13,243,033
Mortgage servicing rights	8,435	8,946
Nonqualified deferred compensation assets	7,951	7,222
Derivative assets	47,964	48,221
FDIC indemnification asset	11,846	85,672
Accrued interest receivable and other	169,156	163,732
Total financial assets	18,370,980	16,937,592
Non-maturity deposits	12,142,034	10,442,077
Deposits with stated maturities	4,139,810	4,226,712
Federal Home Loan Bank advances	733,050	417,762
Other borrowings	196,465	255,104
Subordinated notes	140,000	0
Junior subordinated debentures	249,493	249,493
Derivative liabilities	41,180	37,264
Accrued interest payable	8,001	8,556
Total financial liabilities	17,650,033	15,636,968
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash Equivalents	230,707	263,864
Interest bearing deposits with banks	998,437	495,574
Available-for-sale securities, at fair value	1,792,078	2,176,290
Trading account securities	1,206	497
Federal Home Loan Bank and Federal Reserve Bank stock, at cost	91,582	79,261
Brokerage customer receivables	24,221	30,953
Mortgage loans held-for-sale	351,290	332,485
Mortgage loans held-for-sale, at lower of cost or market	0	1,857
Total Loans	15,346,266	13,867,255
Mortgage servicing rights	8,435	8,946
Nonqualified deferred compensation assets	7,951	7,222
Derivative assets	47,964	48,221
FDIC indemnification asset	11,846	85,672
Accrued interest receivable and other	169,156	163,732
Total financial assets	19,081,139	17,561,829
Non-maturity deposits	12,142,034	10,442,077
Deposits with stated maturities	4,143,161	4,242,172
Federal Home Loan Bank advances	738,113	422,750
Other borrowings	197,883	255,104
Subordinated notes	143,639	0
Junior subordinated debentures	250,305	250,672
Derivative liabilities	41,180	37,264
Accrued interest payable	8,001	8,556
Total financial liabilities	$ 17,664,316	$ 15,658,595