ACNB Corporation (Parent Company Only) Financial Information (Statements of Condition) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Condensed Financial Statements, Captions [Line Items]
Cash	$ 19,078	$ 14,423
Investments in low-income housing partnerships	5,440	3,774
Securities and other assets	14,688	16,010
Total assets	1,049,995	1,004,823	968,667
Long-term Debt	59,954	71,191
Other liabilities	7,298	7,401
Stockholders' equity	101,264	97,474	93,754	88,303
Total Liabilities and Stockholders' Equity	1,049,995	1,004,823
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Cash	1,161	266
Investments in low-income housing partnerships	3,334	3,774
Securities and other assets	1,504	1,634
Receivable from banking subsidiary	985	724
Total assets	103,301	99,665
Long-term Debt	1,954	2,191
Other liabilities	83
Stockholders' equity	101,264	97,474
Total Liabilities and Stockholders' Equity	103,301	99,665
Banking Subsidiary [Member] | Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Investment	85,975	83,274
Other Subsidiary [Member] | Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Investment	$ 10,342	$ 9,993