Earnings per share and shareholders' equity (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Reclassifications out of accumulated other comprehensive income/(loss)
Interest expense	$ 20,304	$ 20,020	$ 59,705	$ 58,758
Amortization of transition obligation, prior service credit and net losses recognized during the period in net periodic benefit cost	(5,789)	(3,826)	(17,490)	(11,467)
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets	5,156	3,342	15,468	10,026
Amount reclassified from AOCI
Reclassifications out of accumulated other comprehensive income/(loss)
Total reclassifications	692	543	1,461	1,537
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)	0	0	(738)	(81)
Interest expense	59	59	177	177
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	5,789	3,826	17,490	11,467
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets	$ (5,156)	$ (3,342)	$ (15,468)	$ (10,026)