Fair Value Measurements Details of Book Value and Fair Value of Financial Instruments (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, FinancialStatement Captions[Line Items]
Available for sale investment securities	$ 2,592,823,000	$ 2,547,956,000
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, FinancialStatement Captions[Line Items]
Cash and due from banks	100,151,000	108,291,000
Interest-bearing deposits with other banks	210,906,000	293,805,000
Federal Reserve Bank and Federal Home Loan Bank stock	56,900,000	60,761,000
Loans held for sale	23,450,000	31,530,000
Available for sale investment securities	2,592,823,000	2,547,956,000
Net Loans	15,533,067,000	15,598,337,000
Accrued interest receivable	53,060,000	52,910,000
Other financial assets	216,952,000	215,464,000
Demand and savings deposits	12,763,521,000	13,042,147,000
Brokered Deposits	64,195,000	90,473,000
Time deposits	2,649,387,000	2,664,912,000
Short-term borrowings	937,852,000	617,524,000
Accrued interest payable	9,681,000	9,317,000
Other financial liabilities	209,483,000	227,569,000
Federal Home Loan Bank advances and long-term debt	938,499,000	1,038,346,000
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, FinancialStatement Captions[Line Items]
Cash and due from banks	100,151,000	108,291,000
Interest-bearing deposits with other banks	210,906,000	293,805,000
Federal Reserve Bank and Federal Home Loan Bank stock	56,900,000	60,761,000
Loans held for sale	23,450,000	31,530,000
Available for sale investment securities	2,592,823,000	2,547,956,000
Net Loans	14,933,445,000	15,380,974,000
Accrued interest receivable	53,060,000	52,910,000
Other financial assets	216,952,000	215,464,000
Demand and savings deposits	12,763,521,000	13,042,147,000
Brokered Deposits	64,195,000	90,473,000
Time deposits	2,647,498,000	2,664,912,000
Short-term borrowings	937,852,000	617,524,000
Accrued interest payable	9,681,000	9,317,000
Other financial liabilities	209,483,000	227,569,000
Federal Home Loan Bank advances and long-term debt	914,763,000	1,038,346,000
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, FinancialStatement Captions[Line Items]
Cash and due from banks	100,151,000	108,291,000
Interest-bearing deposits with other banks	210,906,000	293,805,000
Federal Reserve Bank and Federal Home Loan Bank stock	0	0
Loans held for sale	0	0
Available for sale investment securities	0	918,000
Net Loans	0	0
Accrued interest receivable	53,060,000	52,910,000
Other financial assets	120,156,000	123,439,000
Demand and savings deposits	12,763,521,000	13,042,147,000
Brokered Deposits	64,195,000	90,473,000
Time deposits	0	0
Short-term borrowings	937,852,000	617,524,000
Accrued interest payable	9,681,000	9,317,000
Other financial liabilities	158,232,000	188,555,000
Federal Home Loan Bank advances and long-term debt	0	0
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, FinancialStatement Captions[Line Items]
Cash and due from banks	0	0
Interest-bearing deposits with other banks	0	0
Federal Reserve Bank and Federal Home Loan Bank stock	56,900,000	60,761,000
Loans held for sale	23,450,000	31,530,000
Available for sale investment securities	2,485,814,000	2,444,613,000
Net Loans	0	0
Accrued interest receivable	0	0
Other financial assets	48,304,000	44,539,000
Demand and savings deposits	0	0
Brokered Deposits	0	0
Time deposits	2,647,498,000	2,664,912,000
Short-term borrowings	0	0
Accrued interest payable	0	0
Other financial liabilities	51,251,000	39,014,000
Federal Home Loan Bank advances and long-term debt	914,763,000	1,038,346,000
Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, FinancialStatement Captions[Line Items]
Cash and due from banks	0	0
Interest-bearing deposits with other banks	0	0
Federal Reserve Bank and Federal Home Loan Bank stock	0	0
Loans held for sale	0	0
Available for sale investment securities	107,009,000	102,425,000
Net Loans	14,933,445,000	15,380,974,000
Accrued interest receivable	0	0
Other financial assets	48,492,000	47,486,000
Demand and savings deposits	0	0
Brokered Deposits	0	0
Time deposits	0	0
Short-term borrowings	0	0
Accrued interest payable	0	0
Other financial liabilities	0	0
Federal Home Loan Bank advances and long-term debt	$ 0	$ 0