Summary of Significant Accounting Policies (Narrative) (Details)			2 Months Ended	3 Months Ended	12 Months Ended			36 Months Ended
	Feb. 22, 2021 USD ($) Customer Individual	Oct. 31, 2020 USD ($)	Feb. 18, 2021 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) Apartment shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2020 USD ($) shares	Aug. 26, 2020 shares	Aug. 25, 2020 shares	May 14, 2019 shares	May 13, 2019 shares	Jul. 31, 2018 USD ($)	Jan. 08, 2018 USD ($)	Jun. 25, 2014 shares	May 06, 2014 shares
Cash reserve balances					$ 0	$ 0		$ 0
Net gain on sales of investment securities					467,000	1,878,000	$ 137,000
Loans and leases receivable, net, carrying amount					2,370,659,000	1,753,241,000		2,370,659,000
Loans and leases receivable, gross, carrying amount					2,384,041,000	1,762,756,000	1,624,067,000	2,384,041,000
Loans and leases, net of unearned interest					276,701,000	414,498,000		276,701,000
Bank premises and equipment held for sale					$ 210,000,000	0		210,000,000
Asset held for sale, description					Bank premises and equipment designated as held for sale are carried at the lower of cost or market value, and, at December 31, 2020, totaled $210,000 related to a retail banking property which was closed and listed for sale on December 31, 2020.  There were no premises and equipment classified as held for sale as of December 31, 2019.  During 2019, Mid Penn sold the land and facility formerly used as a full-service retail banking property.
Other securities					$ 1,253,000	$ 782,000	$ 595,000
Federal tax rate					21.00%	21.00%	21.00%
Goodwill					$ 62,840,000	$ 62,840,000		62,840,000
Goodwill, Impairment Loss		$ 0				0	$ 0
Amortization expense					1,398,000	1,430,000	1,224,000
Residential real estate held in other real estate owned					134,000	78,000		134,000
Foreclosure proceedings in process					13,000	84,000		13,000
Mortgage servicing rights					49,000	78,000		49,000
Mortgage servicing rights, amortization					29,000	28,000	20,000
Impairment of mortgage servicing rights								0
Principal balance of loans serviced					9,384,000	12,357,000		9,384,000
Carrying value of investment in limited partnership					$ 146,000	190,000		146,000
Low Income Housing Tax Credit						31,000	$ 76,000
Number of apartments under the project | Apartment					37
Carrying value of investment in limited partnership					$ 146,000	$ 190,000		146,000
Limited partner capital contribution commitment					$ 7,579,000			7,579,000
Project investment amortization period					10 years	10 years
Annual LIHTCs mount awarded for the project					$ 861,000
Total anticipated LIHTCs amount under the housing project					8,613,000			8,613,000
Liabilities of funded status					461,000	$ 183,000		461,000
Settlement gains recognized					3,000	34,000
Trust assets					$ 148,621,000	$ 152,492,000		$ 148,621,000
Common Stock, Shares Authorized | shares					20,000,000	20,000,000		20,000,000	200,000	100,000	20,000,000	10,000,000
Number of antidilutive shares | shares					0	0	0
2014 Restricted Stock Plan [Member]
Shares authorized per plan | shares															200,000	100,000
Other Assets [Member]
Carrying value of investment in limited partnership					$ 6,682,000	$ 7,249,000		$ 6,682,000
Carrying value of investment in limited partnership					$ 6,682,000	7,249,000		6,682,000
Core Deposit Intangible [Member]
Core Deposit Intangible, Amortization Period					10 years
Intangible assets, net					$ 4,311,000	5,526,000		4,311,000
Amortization expense					1,215,000	1,367,000	$ 188,000
Impairment of core deposit intangible					0	0	0
Intangible assets, growth recognized					$ 562,186,000
Finite Lived Intangible Assets Growth Percentage					29.00%
First Priority Financial Corp. [Member]
Goodwill													$ 39,744,000
Goodwill acquired					$ 39,744,000	39,744,000
Scottdale Bank and Trust Company [Member]
Goodwill														$ 19,178,000
Goodwill acquired					19,178,000	19,178,000
Phoenix Bancorp Incorporated [Member]
Goodwill acquired					3,918,000	3,918,000
Federal Home Loan Bank of Pittsburgh [Member]
Other securities					360,000	424,000	$ 275,000
Non Interest Expense [Member]
Asset held for sale, impairment charge				$ 0		105,000
Reserve for Off-balance Sheet Activities [Member]
Valuation allowances and reserves, balance					$ 66,000	80,000		$ 66,000
Subsequent Event [Member]
Loans and leases receivable, gross, carrying amount	$ 290,000,000
Number of business customers for which loan disbursements are made | Customer	2,047
Number of individuals, business customers employ | Individual	26,000
Commercial Portfolio [Member]
Loan terms					1 year
Loan to value ratio					80.00%			80.00%
Small Business Administration [Member] | PPP Loans [Member]
Loan guarantee description					The PPP loans, which are 100 percent guaranteed by the Small Business Administration (“SBA”), have up to a five-year term to maturity and carry a low interest rate of 1 percent throughout the loan term.
Nonrefundable loan processing fees received					$ 20,883,000
Loan processing fees					13,137,000
Loans and leases receivable, net, carrying amount					388,313,000			$ 388,313,000
Loans and leases receivable, gross, carrying amount					396,059,000			396,059,000
Deferred loan processing fees					7,746,000			7,746,000
Loss reserve					0
Small Business Administration [Member] | PPP Loans [Member] | Subsequent Event [Member]
Loan processing fees			$ 12,800,000
Equity Securities [Member]
Equity securities, fair value					515,000	507,000		515,000
Equity Securities [Member] | Accounting Standards Update 2016-01 [Member]
Equity securities, fair value					515,000	507,000		515,000
Equity securities sold						$ 0
Preferred stock of another financial institution, purchased and subsequently sold					1,000,000			$ 1,000,000
Net gain on sales of investment securities					6,000
Equity Securities [Member] | Accounting Standards Update 2016-01 [Member] | Accumulated Other Comprehensive Loss [Member]
Net unrealized losses reclassified					$ 44,000
Maximum [Member]
Maturity of interest bearing time deposits with other financial institutions					1 year
Non-residential consumer loans charged off on contractual basis in event of bankruptcy, in period					120 days			120 days
Maximum [Member] | Building Assets [Member]
Property, plant and equipment, useful life					50 years
Maximum [Member] | Furniture, Fixtures, and Equipment [Member]
Property, plant and equipment, useful life					10 years
Maximum [Member] | Land Improvements [Member]
Property, plant and equipment, useful life					20 years
Maximum [Member] | Leasehold Improvements [Member]
Property, plant and equipment, useful life					15 years
Maximum [Member] | Residential Portfolio [Member]
Loan terms					30 years
Loan to value ratio					100.00%			100.00%
Loan to value ratio, exposure after private mortgage insurance					85.00%
Maximum [Member] | Small Business Administration [Member] | PPP Loans [Member]
Loans maturity term					2 years
Processing fee percentage					5.00%
Maximum [Member] | Home equity lines of credit [Member]
Loan terms					20 years
Loan to value ratio					85.00%			85.00%
Maximum [Member] | Home Equity Lines Of Credit [Member]
Loan terms					5 years
Minimum [Member] | Building Assets [Member]
Property, plant and equipment, useful life					5 years
Minimum [Member] | Furniture, Fixtures, and Equipment [Member]
Property, plant and equipment, useful life					3 years
Minimum [Member] | Land Improvements [Member]
Property, plant and equipment, useful life					10 years
Minimum [Member] | Leasehold Improvements [Member]
Property, plant and equipment, useful life					10 years
Minimum [Member] | Small Business Administration [Member] | PPP Loans [Member]
Processing fee percentage					1.00%
Minimum [Member] | Small Business Administration [Member] | PPP Loans [Member] | Subsequent Event [Member]
Loans and leases receivable, gross, carrying amount			$ 290,000,000