Borrowings - Additional Information (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Long Term And Short Term Debt [Line Items]
Total long-term debt outstanding at a fixed rate	$ 7,300,000
Total long-term debt outstanding at a variable rate	38,100,000
Federal Home Loan Bank Advance
Long Term And Short Term Debt [Line Items]
Total borrowings	581,000,000	$ 587,000,000
Short-term borrowings	535,000,000
Long-term borrowings	45,400,000
Maximum borrowing capacity	2,500,000,000.0
Borrowing capacity utilized	746,000,000
Letter of credit to collateralize public funds	165,000,000
Remaining borrowing capacity	1,700,000,000
Mortgage Backed Securities
Long Term And Short Term Debt [Line Items]
Amortized cost of securities pledged as collateral	52,800,000	57,500,000
Carrying value of securities pledged as collateral	$ 50,900,000	$ 56,800,000

Aggregate borrowing in federal home loan bank.

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- Definition

Long Term And Short Term Debt [Line Items]

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- Definition

Pledged Financial Instruments, Not Separately Reported, Securities, Amortized Cost

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- Definition

Including both current and noncurrent portions, Federal Home Loan Bank borrowings initially due beyond one year or beyond the normal operating cycle if longer.

+ References

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-Publisher FASB
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-SubTopic 210
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- Definition

Amount of advances from Federal Home Loan Bank (FHLBank) maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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- Definition

The portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a set, unchanging rate.

+ References

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The portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a rate subject to change from time to time.

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- Definition

Sum of the carrying amounts as of the date of the latest financial statement presented of securities which are owned but transferred to another party to serve as collateral to partially or fully secure a debt obligation, repurchase agreement or other current or potential obligation, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

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