Subordinated Debt and Trust Preferred Securities (Narrative) (Details) - Subordinated Debt [Member] - USD ($)							3 Months Ended	9 Months Ended
	Nov. 30, 2021	Dec. 22, 2020	Mar. 20, 2020	Dec. 19, 2017	Oct. 01, 2017	Dec. 09, 2015	Mar. 31, 2022	Sep. 30, 2021	Dec. 31, 2021
Subordinated Notes Due December 2030 [Member]
Debt instrument, maturity date								Dec. 31, 2030
Debt instrument, interest rate, effective percentage		4.50%
Debt instrument, Description of variable rate basis								The December 2020 Notes bear interest at a rate of 4.5% per year for the first five years and then float at the Wall Street Journal’s Prime Rate, provided that the interest rate applicable to the outstanding principal balance during the period the December 2020 Notes are floating will at no time be less than 4.5%.
Subordinated debt issuance		$ 12,150,000
Debt instrument, payment terms								Interest is payable quarterly in arrears on March 31, June 30, September 30 and December 31 of each year, beginning on March 31, 2021.
Debt instrument, redemption, description								The December 2020 Notes bear interest at a rate of 4.5% per year for the first five years and then float at the Wall Street Journal’s Prime Rate, provided that the interest rate applicable to the outstanding principal balance during the period the December 2020 Notes are floating will at no time be less than 4.5%. Interest is payable quarterly in arrears on March 31, June 30, September 30 and December 31 of each year, beginning on March 31, 2021.  The December 2020 Notes will mature on December 31, 2030 and are redeemable, in whole or in part, without premium or penalty, on any interest payment date on or after December 31, 2025 and prior to December 31, 2030, subject to any required regulatory approvals.  Additionally, if (i) all or any portion of the December 2020 Notes cease to be deemed Tier 2 Capital, (ii) interest on the December 2020 Notes fails to be deductible for United States federal income tax purposes, or (iii) Mid Penn will be considered an “investment company,” Mid Penn may redeem the December 2020 Notes, in whole but not in part, by giving 10 days’ notice to the holders of the December 2020 Notes.  In the event of a redemption described in the previous sentence, Mid Penn will redeem the December 2020 Notes at 100% of the principal amount of the December 2020 Notes, plus accrued and unpaid interest thereon to but excluding the date of redemption.
Debt instrument, interest rate, effective percentage		4.50%
Notes payable to related parties							$ 750,000		$ 750,000
Subordinated Notes Due March 2030 [Member]
Debt instrument, maturity date							Mar. 30, 2030
Debt instrument, interest rate, effective percentage			4.00%
Debt instrument, Description of variable rate basis							The March 2020 Notes bear interest at a rate of 4.0% per year for the first five years and then float at the Wall Street Journal’s Prime Rate, provided that the interest rate applicable to the outstanding principal balance during the period the March 2020 Notes are floating will at no time be less than 4.25%.
Subordinated debt issuance	$ 6,870,000		$ 15,000,000				$ 8,130,000
Debt instrument, payment terms							Interest is payable semi-annually in arrears on June 30 and December 30 of each year, beginning on June 30, 2020, for the first five years after issuance and will be payable quarterly in arrears thereafter on March 30, June 30, September 30 and December 30.
Debt instrument, redemption, description							The March 2020 Notes bear interest at a rate of 4.0% per year for the first five years and then float at the Wall Street Journal’s Prime Rate, provided that the interest rate applicable to the outstanding principal balance during the period the March 2020 Notes are floating will at no time be less than 4.25%.  Interest is payable semi-annually in arrears on June 30 and December 30 of each year, beginning on June 30, 2020, for the first five years after issuance and will be payable quarterly in arrears thereafter on March 30, June 30, September 30 and December 30.  The March 2020 Notes will mature on March 30, 2030 and are redeemable in whole or in part, without premium or penalty, at any time on or after March 30, 2025 and prior to March 30, 2030.  Additionally, if all or any portion of the March 2020 Notes cease to be deemed Tier 2 Capital, Mid Penn may redeem, on any interest payment date, all or part of the 2020 Notes.  In the event of a redemption described in the previous sentence, Mid Penn will redeem the March 2020 Notes at 100% of the principal amount of the March 2020 Notes, plus accrued and unpaid interest thereon to but excluding the date of redemption.
Debt instrument, interest rate, effective percentage			4.25%
Notes payable to related parties							$ 1,700,000		1,700,000
Subordinated Notes Due 2028 [Member]
Debt instrument, maturity date							Jan. 01, 2028
Debt instrument, interest rate, effective percentage				5.25%
Debt instrument, Description of variable rate basis							The 2017 Notes bear interest at a rate of 5.25% per year for the first five years and then float at the Wall Street Journal’s Prime Rate plus 0.50%, provided that the interest rate applicable to the outstanding principal balance will at no time be less than 5.0%.
Subordinated debt issuance				$ 10,000,000
Debt instrument, payment terms							Interest is payable semi-annually in arrears on January 15 and July 15 of each year, beginning on July 15, 2018, for the first five years after issuance and will be payable quarterly in arrears thereafter on January 15, April 15, July 15, and October 15.
Debt instrument, redemption, description							The 2017 Notes bear interest at a rate of 5.25% per year for the first five years and then float at the Wall Street Journal’s Prime Rate plus 0.50%, provided that the interest rate applicable to the outstanding principal balance will at no time be less than 5.0%. Interest is payable semi-annually in arrears on January 15 and July 15 of each year, beginning on July 15, 2018, for the first five years after issuance and will be payable quarterly in arrears thereafter on January 15, April 15, July 15, and October 15. The 2017 Notes will mature on January 1, 2028 and are redeemable in whole or in part, without premium or penalty, at any time on or after December 21, 2022, and prior to January 1, 2028. Additionally, Mid Penn may redeem the 2017 Notes in whole at any time, or in part from time to time, upon at least 30 days’ notice if: (i) a change or prospective change in law occurs that could prevent Mid Penn from deducting interest payable on the 2017 Notes for U.S. federal income tax purposes; (ii) an event occurs that precludes the 2017 Notes from being recognized as Tier 2 capital for regulatory capital purposes; or (iii) Mid Penn becomes required to register as an investment company under the Investment Company Act of 1940, as amended. In the event of a redemption described in the previous sentence, Mid Penn will redeem the 2017 Notes at 100% of the principal amount of the 2017 Notes, plus accrued and unpaid interest thereon to but excluding the date of redemption.
Notes payable to related parties							$ 1,450,000		1,450,000
Unamortized debt issuance cost							$ 38,000		44,000
Subordinated Notes Due 2028 [Member] | WSJ Prime Rate [Member]
Debt instrument, basis spread on variable rate				0.50%
Subordinated Notes Due 2028 [Member] | WSJ Prime Rate [Member] | Maximum [Member]
Debt instrument, interest rate, effective percentage				5.00%
Subordinated Notes Due 2025 [Member]
Debt instrument, maturity date							Dec. 09, 2025
Debt instrument, interest rate, effective percentage						5.15%
Debt instrument, Description of variable rate basis							The 2015 Notes paid interest at a rate of 5.15% per year for the first five years outstanding, including the three months ended March 31, 2020. Beginning January 1, 2021, the 2015 Notes bear interest at a floating rate based on the Wall Street Journal’s Prime Rate plus 0.50%, provided that the interest rate applicable to the outstanding principal balance will at no time be less than 4.0%.
Subordinated debt issuance						$ 7,500,000
Debt instrument, payment terms							Interest is payable quarterly in arrears on January 1, April 1, July 1 and October 1 of each year, beginning on January 1, 2016.
Debt instrument, redemption, description							The 2015 Notes paid interest at a rate of 5.15% per year for the first five years outstanding, including the three months ended March 31, 2020. Beginning January 1, 2021, the 2015 Notes bear interest at a floating rate based on the Wall Street Journal’s Prime Rate plus 0.50%, provided that the interest rate applicable to the outstanding principal balance will at no time be less than 4.0%.  Interest is payable quarterly in arrears on January 1, April 1, July 1 and October 1 of each year, beginning on January 1, 2016. The 2015 Notes will mature on December 9, 2025 and are redeemable in whole or in part, without premium or penalty, at any time on or after December 9, 2020, and prior to December 9, 2025.  Additionally, Mid Penn may redeem the 2015 Notes in whole at any time, or in part from time to time, upon at least 30 days’ notice if:  (i) a change or prospective change in law occurs that could prevent Mid Penn from deducting interest payable on the 2015 Notes for U.S. federal income tax purposes; (ii) an event occurs that precludes the 2015 Notes from being recognized as Tier 2 capital for regulatory capital purposes; or (iii) Mid Penn becomes required to register as an investment company under the Investment Company Act of 1940, as amended, in each case at 100% of the principal amount of the 2015 Notes, plus accrued and unpaid interest thereon to but excluding the date of redemption.
Notes payable to related parties							$ 1,930,000		1,930,000
Unamortized debt issuance cost							$ 38,000		$ 44,000
Subordinated Notes Due 2025 [Member] | WSJ Prime Rate [Member]
Debt instrument, basis spread on variable rate						0.50%
Subordinated Notes Due 2025 [Member] | WSJ Prime Rate [Member] | Maximum [Member]
Debt instrument, interest rate, effective percentage						4.00%
Riverview Acquisition [Member]
Subordinate debt assumed	25,000,000
Subordinated debt fair value premium	$ 2,302,000
Riverview agreement date	Oct. 06, 2020
Debt instrument, maturity date	Oct. 15, 2030
Debt instrument, interest rate, effective percentage	5.75%
Debt instrument, Description of variable rate basis	quarterly to an interest rate per annum equal to the then current three-month secured overnight financing rate (“SOFR”) plus 563 basis points, payable quarterly until maturity. Mid Penn may redeem the Notes at par, in whole or in part, at its option, anytime beginning on October 15, 2025
CBT 2017 Notes [Member]
Debt instrument acquisition date					Oct. 01, 2017
Subordinated debentures issued					5,155,000
Debt instrument face value percentage					100.00%
CBT 2017 Notes [Member] | Three-Month LIBOR Rate Plus 2.95% [Member]
Debt instrument, interest rate, effective percentage					2.95%
CBT 2015 Notes [Member]
Subordinated debentures issued					4,124,000
Debt instrument face value percentage					100.00%
CBT 2017 Notes and CBT 2015 Notes [Member]
Debt instrument fair value premium					$ 6,000