Summary of significant accounting policies (Details 3) (USD $)	3 Months Ended								12 Months Ended								1 Months Ended	12 Months Ended			12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Electric utility	Dec. 31, 2010 Electric utility	Dec. 31, 2009 Electric utility	Dec. 31, 2011 Electric utility Minimum Y	Dec. 31, 2011 Electric utility Maximum Y	Dec. 31, 2008 Bank	Dec. 31, 2011 Bank M D	Sep. 30, 2011 Bank	Dec. 31, 2010 Bank	Dec. 31, 2011 Bank Minimum D	Dec. 31, 2011 Bank Maximum D	Dec. 31, 2010 Stock appreciation rights (SARs) Common stock.
Basic
Distributed earnings (in dollars per share)									$ 1.24	$ 1.24	$ 1.24
Undistributed earnings (losses) (in dollars per share)									$ 0.21	$ (0.02)	$ (0.33)
Earnings Per Share, basic (in dollars per share)	$ 0.36	$ 0.50	$ 0.28	$ 0.30	$ 0.26	$ 0.35	$ 0.31	$ 0.29	$ 1.45	$ 1.22	$ 0.91
Diluted
Distributed earnings (in dollars per share)									$ 1.24	$ 1.24	$ 1.24
Undistributed earnings (losses) (in dollars per share)									$ 0.20	$ (0.03)	$ (0.33)
Earnings Per Share, diluted (in dollars per share)	$ 0.36	$ 0.50	$ 0.28	$ 0.30	$ 0.26	$ 0.35	$ 0.31	$ 0.29	$ 1.44	$ 1.21	$ 0.91
Summary of significant accounting policies
Antidilutive securities excluded from computation of diluted earnings per share (in shares)																							450,000
Contributions in aid of construction
Contributions amortized period (in years)															30	51
Electric utility revenues
Revenue taxes included in "operating revenues" and in "taxes, other than income taxes" expense												$ 264,000,000	$ 211,000,000	$ 181,000,000
Weighted average AFUDC rate (as a percent)												8.00%	8.10%	8.10%
Allowance for loan losses
Past due period after which the accrual of interest on loans ceases (in days)																		90
Past due unsecured consumer loans, period to reach delinquency status (in days)																					120	180
Loans modified in a troubled debt restructuring
Number of consecutive months of repayment required for loans to be removed from nonaccrual status																		6
Real estate acquired in settlement of loans
Real estate acquired in settlement of loans																		7,300,000		4,300,000
Goodwill and other intangibles
Goodwill	82,190,000				82,190,000				82,190,000	82,190,000								82,200,000		82,200,000
Goodwill written off related to sale of business of Bishop Insurance Agency																	$ 900,000
Estimated fair value in excess of book value (as a percent)																			35.00%
Probability threshold for goodwill impairment (as a percent)																		50.00%