Investment Securities (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Investment Securities [Abstract]
Available-for-sale Securities Pledged as Collateral	$ 95,258,000		$ 95,258,000		$ 85,591,000
Available-for-sale Securities, Gross Realized Gains	241,000	0	267,000	0
Available-for-sale Securities, Gross Realized Losses	$ 0	$ 0	$ 0	$ 0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	69		69		45
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Severity			At September 30, 2012, Mid Penn had 69 debt securities from separate issuers with unrealized losses. These securities have depreciated .97% from their amortized cost basis.		At December 31, 2011, 45 debt securities with unrealized losses had depreciated 1.37% from the amortized cost basis.
Average Life of Mortgage-backed Securities	1 year 10 months 24 days		1 year 10 months 24 days