Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Statement of Other Comprehensive Income [Abstract]
Net income for common stock	$ 48,236	$ 47,706	$ 122,503	$ 124,822
Net unrealized gains (losses) on securities:
Net unrealized gains (losses) on securities arising during the period, net of (taxes) benefits of $1,049 and ($689) for the three months ended September 30, 2013 and 2012 and $7,081 and ($1,261) for the nine months ended September 30, 2013 and 2012, respectively	(1,589)	1,043	(10,724)	1,910
Less: reclassification adjustment for net realized gains included in net income, net of taxes of nil for the three months ended September 30, 2013 and 2012 and $488 and $53 for the nine months ended September 30, 2013 and 2012, respectively	0	0	(738)	(81)
Derivatives qualified as cash flow hedges:
Less: reclassification adjustment to net income, net of tax benefits of $37 for the three months ended September 30, 2013 and 2012 and $112 for the nine months ended September 30, 2013 and 2012	59	59	177	177
Retirement benefit plans:
Less: amortization of transition obligation, prior service credit and net losses recognized during the period in net periodic benefit cost, net of tax benefits of $3,697 and $2,443 for the three months ended September 30, 2013 and 2012 and $11,173 and $7,321 for the nine months ended September 30, 2013 and 2012, respectively	5,789	3,826	17,490	11,467
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, net of taxes of $3,284 and $2,129 for the three months ended September 30, 2013 and 2012 and $9,852 and $6,386 for the nine months ended September 30, 2013 and 2012, respectively	(5,156)	(3,342)	(15,468)	(10,026)
Other comprehensive income (loss), net of taxes	(897)	1,586	(9,263)	3,447
Comprehensive income	$ 47,339	$ 49,292	$ 113,240	$ 128,269