Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement of Comprehensive Income [Abstract]
Net income for common stock	$ 41,421	$ 40,588	$ 87,348	$ 74,267
Net unrealized gains (losses) on securities:
Net unrealized gains (losses) on securities arising during the period, net of (taxes) benefits of ($1,679), $5,485, ($3,343) and $6,032 for the respective periods	2,543	(8,307)	5,063	(9,135)
Less: reclassification adjustment for net realized gains included in net income, net of taxes of nil, $488, $1,132 and $488 for the respective periods	0	(738)	(1,715)	(738)
Derivatives qualified as cash flow hedges:
Less: reclassification adjustment to net income, net of tax benefits of $38, $38, $75 and $75 for the respective periods	59	59	118	118
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 $1,742, $3,630, $3,632 and $7,476 for the respective periods	2,873	5,680	5,686	11,701
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, net of taxes of $1,641, $3,184, $3,239 and $6,568 for the respective periods	(2,575)	(4,999)	(5,085)	(10,312)
Other comprehensive income (loss), net of taxes	2,900	(8,305)	4,067	(8,366)
Comprehensive income	$ 44,321	$ 32,283	$ 91,415	$ 65,901