Fair Value Measurements Details of Book Value and Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, FinancialStatement Captions[Line Items]
Available for sale investment securities	$ 2,516,205	$ 2,484,773
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, FinancialStatement Captions[Line Items]
Cash and Cash Equivalents	83,479	101,120
Interest-Rate Deposits with Other Banks	346,582	230,300
Investment in Federal Home Loan Bank Stock, Fair Value Disclosure	61,478	62,216
Loans Held-for-sale	19,719	16,886
Available for sale investment securities	2,516,205	2,484,773
Net Loans	13,706,860	13,669,548
Accrued Interest Receivable	44,379	42,767
Other Financial Assets	200,456	166,920
Demand and Savings Deposit Liabilities, Fair Value Disclosure	11,537,266	11,267,367
Time Deposit Liabilities, Fair Value Disclosure	2,867,014	2,864,950
Short-term borrowings	352,883	497,663
Accrued interest payable	13,567	10,724
Other Financial Liabilities	220,267	190,927
Federal Home Loan Bank Advances And Debt Long Term, Fair Value Disclosre	965,654	949,542
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, FinancialStatement Captions[Line Items]
Cash and Cash Equivalents	83,479	101,120
Interest-Rate Deposits with Other Banks	346,582	230,300
Investment in Federal Home Loan Bank Stock, Fair Value Disclosure	61,478	62,216
Loans Held-for-sale	19,719	16,886
Available for sale investment securities	2,516,205	2,484,773
Net Loans	13,615,387	13,540,903
Accrued Interest Receivable	44,379	42,767
Other Financial Assets	200,456	166,920
Demand and Savings Deposit Liabilities, Fair Value Disclosure	11,537,266	11,267,367
Time Deposit Liabilities, Fair Value Disclosure	2,961,225	2,862,868
Short-term borrowings	352,883	497,663
Accrued interest payable	13,567	10,724
Other Financial Liabilities	220,267	190,927
Federal Home Loan Bank Advances And Debt Long Term, Fair Value Disclosre	$ 990,222	$ 959,315