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Bank Premises and Equipment (Narrative) (Detail)
 6 Months Ended 12 Months Ended
Jan. 20, 2017
USD ($)
Property
Jun. 30, 2017
USD ($)
Jun. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Lease
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Property, Plant and Equipment [Line Items]            
Depreciation   $ 715,000 $ 820,000 $ 1,658,000 $ 1,485,000 $ 1,235,000
Asset held for sale, description       As of December 31, 2016, assets held-for-sale consisted of three full service retail banking properties. These properties were transferred from land and buildings to assets held for sale due to Mid Penn’s intent to sell them during January 2017    
Number of properties sale | Property 3          
Proceeds from sale of three properties $ 2,240,000 2,201,000        
Gain from sale of three properties $ 346,000 $ (26,000)   $ (142,000)   (18,000)
2017       1,121,000    
2018       1,115,000    
2019       1,116,000    
2020       793,000    
2021       582,000    
thereafter       $ 5,666,000    
Lease expiration date       Dec. 31, 2035    
Sublease to Unrelated Party [Member]            
Property, Plant and Equipment [Line Items]            
Number of operating lease with related parties | Lease       2    
Lease expiration date       Mar. 31, 2021    
Operating Leases Related Party [Member]            
Property, Plant and Equipment [Line Items]            
Number of operating lease with related parties | Lease       4    
Operating leases, rent expense, net       $ 348,000 $ 279,000 $ 54,000
2017       346,000    
2018       302,000    
2019       304,000    
2020       305,000    
2021       213,000    
thereafter       $ 2,118,000    
Second Sublease Agreement [Member] | Sublease to Unrelated Party [Member]            
Property, Plant and Equipment [Line Items]            
Sale and lease back of properties, description Two of the properties are being leased back by Mid Penn for a period of at least 15 years, and the respective gains on the sales of those properties will be recognized over the life of the leases.          
Lease agreement start date       Jul. 01, 2016    
First Sublease Agreement [Member] | Sublease to Unrelated Party [Member]            
Property, Plant and Equipment [Line Items]            
Lease agreement start date       Apr. 01, 2016    
Minimum [Member]            
Property, Plant and Equipment [Line Items]            
Length Of Lease 15 years          
Other Expense [Member]            
Property, Plant and Equipment [Line Items]            
Asset held for sale, impairment charge on one of the properties       $ 142,000