Basis of Presentation (Policies)	12 Months Ended
Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Use of estimates	Use of estimates: We must make estimates and assumptions that affect the reported amounts and disclosures in preparing our financial statements in conformity with accounting principles generally accepted in the United States of America. Actual results could differ significantly from those estimates.
Principles of consolidation	Principles of consolidation: The accompanying consolidated financial statements include the accounts of Summit and its wholly-owned subsidiaries. All significant accounts and transactions among these entities have been eliminated.
Comprehensive Income, Policy	Comprehensive income: Comprehensive income consists of net income and other comprehensive income. Other comprehensive income includes unrealized gains and losses on securities available for sale, cash flow hedges and other post-retirement benefits, which are recognized as separate components of equity.
Cash and cash equivalents	Cash and cash equivalents: Cash and cash equivalents includes cash on hand, amounts due from banks (including cash items in process of clearing), interest bearing deposits with other banks and federal funds sold.
Finance, Loan and Lease Receivables, Held-for-sale, Policy	Loans held for sale: Loans held for sale are valued at the lower of aggregate carrying cost or fair value. Gains or losses realized on the sales of loans are recognized in other income at the time of sale.
Cash surrender value of life insurance policies	Cash surrender value of life insurance policies: We have purchased life insurance policies on certain employees. Company owned life insurance is recorded at the amount that can be realized under the insurance contract at the balance sheet date, which is the cash surrender value adjusted for other charges or other amounts due that are probable at settlement.
Presentation of cash flows	Presentation of cash flows: For purposes of reporting, cash flows from demand deposits, NOW accounts, savings accounts and short-term borrowings are reported on a net basis, since their original maturities are less than three months. Cash flows from loans and certificates of deposit and other time deposits are reported net.
Advertising	Advertising: Advertising costs are expensed as incurred
Trust services	Trust services: Assets held in an agency or fiduciary capacity are not our assets and are not included in the accompanying consolidated balance sheets. Trust services income is recognized on the cash basis in accordance with customary banking practice. Reporting such income on a cash basis does not produce results that are materially different from those that would result from use of the accrual basis.
Variable interest entities	Unconsolidated subsidiary trusts: In accordance with accounting principles generally accepted in the United States, we do not consolidate subsidiary trusts which issue guaranteed preferred beneficial interests in subordinated debentures (Trust Preferred Securities). The Trust Preferred Securities qualify as Tier 1 capital for regulatory purposes. See Note 13 of our Notes to Consolidated Financial Statements for a discussion of our subordinated debentures owed to unconsolidated subsidiary trusts.

Basis of Presentation (Policies)

12 Months Ended

Dec. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Use of estimates: We must make estimates and assumptions that affect the reported amounts and disclosures in preparing our financial statements in conformity with accounting principles generally accepted in the United States of America. Actual results could differ significantly from those estimates.

Principles of consolidation

Principles of consolidation: The accompanying consolidated financial statements include the accounts of Summit and its wholly-owned subsidiaries. All significant accounts and transactions among these entities have been eliminated.

Comprehensive Income, Policy

Comprehensive income: Comprehensive income consists of net income and other comprehensive income. Other comprehensive income includes unrealized gains and losses on securities available for sale, cash flow hedges and other post-retirement benefits, which are recognized as separate components of equity.

Cash and cash equivalents

Cash and cash equivalents: Cash and cash equivalents includes cash on hand, amounts due from banks (including cash items in process of clearing), interest bearing deposits with other banks and federal funds sold.

Finance, Loan and Lease Receivables, Held-for-sale, Policy

Loans held for sale: Loans held for sale are valued at the lower of aggregate carrying cost or fair value. Gains or losses realized on the sales of loans are recognized in other income at the time of sale.

Cash surrender value of life insurance policies

Cash surrender value of life insurance policies: We have purchased life insurance policies on certain employees. Company owned life insurance is recorded at the amount that can be realized under the insurance contract at the balance sheet date, which is the cash surrender value adjusted for other charges or other amounts due that are probable at settlement.

Presentation of cash flows

Presentation of cash flows: For purposes of reporting, cash flows from demand deposits, NOW accounts, savings accounts and short-term borrowings are reported on a net basis, since their original maturities are less than three months. Cash flows from loans and certificates of deposit and other time deposits are reported net.

Advertising

Advertising: Advertising costs are expensed as incurred

Trust services

Trust services: Assets held in an agency or fiduciary capacity are not our assets and are not included in the accompanying consolidated balance sheets. Trust services income is recognized on the cash basis in accordance with customary banking practice. Reporting such income on a cash basis does not produce results that are materially different from those that would result from use of the accrual basis.

Variable interest entities

Unconsolidated subsidiary trusts: In accordance with accounting principles generally accepted in the United States, we do not consolidate subsidiary trusts which issue guaranteed preferred beneficial interests in subordinated debentures (Trust Preferred Securities). The Trust Preferred Securities qualify as Tier 1 capital for regulatory purposes. See Note 13 of our Notes to Consolidated Financial Statements for a discussion of our subordinated debentures owed to unconsolidated subsidiary trusts.