Short-Term Borrowings (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Short-term borrowings	$ 693,807	$ 603,487
Federal Home Loan Bank And Federal Reserve Bank Amount Of Available Unused Funds	7,330,000
Repurchase Agreements [Member]
Short-term Debt [Line Items]
Short-term borrowings	586,171	590,772
Federal Home Loan Bank Advances [Member]
Short-term Debt [Line Items]
Short-term borrowings	90,000	10,000
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Short-term borrowings	2,551	2,551
Subordinated Debt Obligations [Member]
Short-term Debt [Line Items]
Short-term borrowings	15,000	0
Unamortized Purchase Accounting Adjustments [Member]
Short-term Debt [Line Items]
Short-term borrowings	85	$ 164
Unsecured Debt [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 665,000