Fair Value Measurements Details of Book Value and Fair Value of Financial Instruments (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, FinancialStatement Captions[Line Items]
Securities held to maturity	$ 622,852,000
Estimated Fair Value	2,008,816,000
Available for sale investment securities		$ 2,547,956,000
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, FinancialStatement Captions[Line Items]
Cash and due from banks	90,361,000	108,291,000
Interest-bearing deposits with other banks	322,330,000	293,805,000
Federal Reserve Bank and Federal Home Loan Bank stock	65,926,000	60,761,000
Loans held for sale	27,525,000	31,530,000
Securities held to maturity	626,597,000
Estimated Fair Value	2,008,816,000
Available for sale investment securities		2,547,956,000
Net Loans	15,767,283,000	15,598,337,000
Accrued interest receivable	58,584,000	52,910,000
Other financial assets	242,140,000	215,464,000
Demand and savings deposits	13,384,227,000	13,042,147,000
Brokered deposits	164,601,000	90,473,000
Time deposits	2,700,186,000	2,664,912,000
Short-term borrowings	485,565,000	617,524,000
Accrued interest payable	11,151,000	9,317,000
Other financial liabilities	251,460,000	227,569,000
Federal Home Loan Bank advances and long-term debt	992,115,000	1,038,346,000
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, FinancialStatement Captions[Line Items]
Cash and due from banks	90,361,000	108,291,000
Interest-bearing deposits with other banks	322,330,000	293,805,000
Federal Reserve Bank and Federal Home Loan Bank stock	65,926,000	60,761,000
Loans held for sale	27,525,000	31,530,000
Securities held to maturity	622,852,000
Estimated Fair Value	2,008,816,000
Available for sale investment securities		2,547,956,000
Net Loans	15,124,455,000	15,380,974,000
Accrued interest receivable	58,584,000	52,910,000
Other financial assets	242,139,000	215,464,000
Demand and savings deposits	13,384,227,000	13,042,147,000
Brokered deposits	164,601,000	90,473,000
Time deposits	2,684,783,000	2,664,912,000
Short-term borrowings	485,565,000	617,524,000
Accrued interest payable	11,151,000	9,317,000
Other financial liabilities	251,460,000	227,569,000
Federal Home Loan Bank advances and long-term debt	960,168,000	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	90,361,000	108,291,000
Interest-bearing deposits with other banks	322,330,000	293,805,000
Federal Reserve Bank and Federal Home Loan Bank stock	0	0
Loans held for sale	0	0
Securities held to maturity	622,852,000
Estimated Fair Value	0
Available for sale investment securities		918,000
Net Loans	0	0
Accrued interest receivable	58,584,000	52,910,000
Other financial assets	126,148,000	123,439,000
Demand and savings deposits	13,384,227,000	13,042,147,000
Brokered deposits	164,601,000	90,473,000
Time deposits	0	0
Short-term borrowings	485,565,000	617,524,000
Accrued interest payable	11,151,000	9,317,000
Other financial liabilities	176,429,000	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	65,926,000	60,761,000
Loans held for sale	27,525,000	31,530,000
Securities held to maturity	0
Estimated Fair Value	1,902,333,000
Available for sale investment securities		2,444,613,000
Net Loans	0	0
Accrued interest receivable	0	0
Other financial assets	67,079,000	44,539,000
Demand and savings deposits	0	0
Brokered deposits	0	0
Time deposits	2,684,783,000	2,664,912,000
Short-term borrowings	0	0
Accrued interest payable	0	0
Other financial liabilities	75,031,000	39,014,000
Federal Home Loan Bank advances and long-term debt	960,168,000	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
Securities held to maturity	0
Estimated Fair Value	106,483,000
Available for sale investment securities		102,425,000
Net Loans	15,124,455,000	15,380,974,000
Accrued interest receivable	0	0
Other financial assets	48,912,000	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