Earnings per share and shareholders' equity (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Reclassifications out of accumulated other comprehensive income
Interest expense	$ 19,788	$ 18,539
Amortization of transition obligation, prior service credit and net losses recognized during the period in net periodic benefit cost	(6,021)	(3,873)
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets	5,313	3,395
Amount reclassified from AOCI
Reclassifications out of accumulated other comprehensive income
Total reclassifications	767	537
Derivatives qualified as cash flow hedges Interest rate contracts (settled in 2011) | Amount reclassified from AOCI
Reclassifications out of accumulated other comprehensive income
Interest expense	59	59
Retirement benefit plan items | Amount reclassified from AOCI
Reclassifications out of accumulated other comprehensive income
Amortization of transition obligation, prior service credit and net losses recognized during the period in net periodic benefit cost	6,021	3,873
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets	$ (5,313)	$ (3,395)