Shareholders' Equity (Narrative) (Details)		9 Months Ended	12 Months Ended
	Sep. 14, 2016 USD ($) $ / shares	Sep. 30, 2016	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Oct. 16, 2015 $ / shares	Sep. 24, 2013 shares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
number of years			2
Dividends declared to shareholders			$ 14,600,000	$ 12,900,000	$ 10,600,000
Stockholders' Equity Note, Stock Split Ratio, Numerator | shares				1
Stockholders' Equity Note, Stock Split Ratio, Denominator | shares				2
Share Price | $ / shares	$ 31.75
Stockholders' Equity Note, Stock Split, Reduction of Capital	$ 5,000			$ (5,000)
Stockholders' Equity Note, Stock Split, Conversion Ratio		1.5	1.5
Acadia Trust [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Dividends declared to shareholders					0
Subsidiaries [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Dividends declared to shareholders			$ 16,800,000	$ 16,000,000	$ 39,200,000
2012 Common Stock Repurchase Program [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock repurchase program, shares of common stock approved for repurchase | shares							375,000
Stock repurchased during period, shares | shares			0	374,250
Shares repurchased, average cost per share | $ / shares			$ 26.55
The percentage of the stock repurchase program's allotment repurchased during the period			99.80%
SBM Financial, Inc. [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Share Price | $ / shares						$ 26.32
SBM Financial, Inc. [Member] | Subsidiaries [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Dividends declared to shareholders				$ 30,000,000