Income taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Federal
Current	$ (7,638,000)	$ (25,446,000)	$ 25,691,000
Deferred	73,494,000	85,268,000	14,161,000
Deferred tax credits, net		(901,000)	(593,000)
Federal taxes	65,856,000	58,921,000	39,259,000
State
Current	2,437,000	(7,392,000)	6,930,000
Deferred	5,949,000	13,425,000	(783,000)
Deferred tax credits, net	1,690,000	2,868,000	(1,483,000)
State taxes	10,076,000	8,901,000	4,664,000
Federal and state taxes	75,932,000	67,822,000	43,923,000
Reconciliation percentage of amount of income taxes computed at federal statutory rate	35.00%	35.00%	35.00%
Amount at the federal statutory income tax rate	75,618,000	64,136,000	45,088,000
Increase (decrease) resulting from:
State income taxes, net of effect on federal income taxes	6,550,000	5,786,000	3,033,000
Other, net	(6,236,000)	(2,100,000)	(4,198,000)
Federal and state taxes	75,932,000	67,822,000	43,923,000
Effective income tax rate (as a percent)	35.10%	37.00%	34.10%
Deferred tax assets
Allowance for loan losses	14,076,000	16,461,000
Retirement benefits	6,175,000
Other	33,217,000	35,878,000
Total deferred tax assets	53,468,000	52,339,000
Deferred tax liabilities
Property, plant and equipment	255,488,000	188,490,000
Retirement benefits		2,479,000
Goodwill	22,028,000	20,130,000
Regulatory assets, excluding amounts attributable to property, plant and equipment	32,343,000	32,074,000
FHLB stock dividend	20,552,000	20,552,000
Change in accounting method related to repairs	48,566,000	46,702,000
Other	28,542,000	20,870,000
Total deferred tax liabilities	407,519,000	331,297,000
Net deferred income tax liability	354,051,000	278,958,000
Interest income on income tax refunds		9,700,000
Interest expense/(credit adjustments to interest expense) on income taxes	(1,200,000)	(900,000)	700,000
Amount of accrued interest related to uncertain tax positions	1,500,000	2,700,000
Portion of liability for uncertain tax positions, if recognized, would affect the Company's effective tax rate	900,000
Estimated decrease in the liability for uncertain tax positions within the next 12 months which is reasonably possible	5,000,000
Changes in total unrecognized tax benefits
Unrecognized tax benefits, at the beginning of the period	15,400,000	26,500,000	27,900,000
Additions based on tax positions taken during the year		11,000,000
Reductions based on tax positions taken during the year	(600,000)
Additions for tax positions of prior years	100,000	2,200,000	400,000
Reductions for tax positions of prior years	(8,100,000)	(18,200,000)	(1,800,000)
Settlements		(6,100,000)
Lapses of statute of limitations	(1,100,000)
Unrecognized tax benefits, at the end of the period	$ 5,700,000	$ 15,400,000	$ 26,500,000