Summary of Significant Accounting Policies (Narrative) (Details)			3 Months Ended				12 Months Ended
	Oct. 31, 2019 USD ($)	Jan. 02, 2019 USD ($)	Mar. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($) shares	Jun. 30, 2018 USD ($) Apartment	Dec. 31, 2019 USD ($)	Jan. 01, 2019 USD ($) Branch
Loans and leases, net of unearned interest			$ 373,225,000	$ 414,498,000			$ 414,498,000
Bank premises and equipment held for sale			$ 0	0			0
Asset held for sale, description			Bank premises and equipment designated as held for sale are carried at the lower of cost or market value. There were no premises and equipment classified as held for sale as of March 31, 2020 or December 31, 2019. During 2019, Mid Penn sold the land and facility formerly used as a full-service retail banking property.
Residential real estate held in other real estate owned			$ 135,000	78,000			78,000
Foreclosure proceedings in process			97,000	84,000			84,000
Operating lease right of use asset			10,999,000	11,442,000			11,442,000
Operating lease liability			12,070,000	12,544,000			12,544,000
Carrying value of investment in limited partnership			179,000	$ 190,000			$ 190,000
Number of apartments under the project | Apartment						37
Limited partner capital contribution commitment						$ 7,579,000
Project investment amortization period				10 years		10 years	10 years
Annual LIHTCs mount awarded for the project						$ 861,000
Total anticipated LIHTCs amount under the housing project						$ 8,613,000
Amortization expense			323,000		$ 363,000
Goodwill			62,840,000	$ 62,840,000			$ 62,840,000
Goodwill, Impairment Loss	$ 0		0
Decrease in fair value of plan assets			(535,000)
Decrease in accumulated other comprehensive (loss) income, net of taxes			$ (535,000)
Number of antidilutive shares | shares			0		0
First Priority Financial Corp. [Member]
Goodwill acquired			$ 39,744,000				39,744,000
Scottdale Bank and Trust Company [Member]
Goodwill acquired			19,178,000				19,178,000
Phoenix Bancorp Incorporated [Member]
Goodwill acquired			3,918,000				3,918,000
Core Deposit Intangible [Member]
Intangible assets, net			5,215,000	5,526,000			5,526,000
Amortization expense			$ 310,000		$ 348,000
Core Deposit Intangible, Amortization Period			10 years
Impairment of core deposit intangible			$ 0
Intangible assets, growth recognized			$ 60,887,000
Finite Lived Intangible Assets Growth Percentage			3.20%
Other Assets [Member]
Carrying value of investment in limited partnership			$ 7,250,000	7,249,000			7,249,000
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			$ 80,000	80,000			80,000
Maximum [Member]
Non-residential consumer loans charged off on contractual basis in event of bankruptcy, in period			120 days
Commercial Portfolio [Member]
Loan terms			1 year
Loan to value ratio			80.00%
Residential Portfolio [Member] | Maximum [Member]
Loan terms			30 years
Loan to value ratio			100.00%
Loan to value ratio, exposure after private mortgage insurance			85.00%
ASU 842 [Member]
Operating lease right of use asset								$ 11,661,000
Operating lease liability								$ 12,866,000
opening adjustment to retained earnings		$ 316,000
Number of retail branch location | Branch								2
Equity Securities [Member]
Equity securities, fair value			$ 515,000	507,000			507,000
Equity Securities [Member] | ASU 2016-01 [Member]
Equity securities, fair value			515,000	$ 507,000			$ 507,000
Equity securities sold			$ 0		$ 0
Home equity lines of credit [Member] | Maximum [Member]
Loan terms			20 years
Loan to value ratio			85.00%
Home Equity Lines Of Credit [Member] | Maximum [Member]
Loan terms			5 years