Earnings per share and shareholders' equity (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Reclassifications out of accumulated other comprehensive income/(loss)
Interest expense	$ 19,613	$ 20,199	$ 39,401	$ 38,738
Amortization of transition obligation, prior service credit and net losses recognized during the period in net periodic benefit cost	(5,680)	(3,768)	(11,701)	(7,641)
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets	4,999	3,289	10,312	6,684
Amount reclassified from AOCI
Reclassifications out of accumulated other comprehensive income/(loss)
Total reclassifications	2	457	769	994
Derivatives qualified as cash flow hedges Interest rate contracts (settled in 2011) | Amount reclassified from AOCI
Reclassifications out of accumulated other comprehensive income/(loss)
Revenues-bank (net gains on sales of securities)	(738)	(81)	(738)	(81)
Interest expense	59	59	118	118
Retirement benefit plan items | Amount reclassified from AOCI
Reclassifications out of accumulated other comprehensive income/(loss)
Amortization of transition obligation, prior service credit and net losses recognized during the period in net periodic benefit cost	5,680	3,768	11,701	7,641
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets	$ (4,999)	$ (3,289)	$ (10,312)	$ (6,684)