Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Statement of Comprehensive Income [Abstract]
Net income for common stock	$ 47,815	$ 48,236	$ 135,163	$ 122,503
Net unrealized gains (losses) on securities:
Net unrealized gains (losses) on securities arising during the period, net of (taxes) tax benefits of $1,094, $1,049, ($2,249) and $7,081 for the respective periods	(1,657)	(1,589)	3,406	(10,724)
Less: reclassification adjustment for net realized gains included in net income, net of taxes of nil, nil, $1,132 and $488 for the respective periods	0	0	(1,715)	(738)
Derivatives qualified as cash flow hedges:
Less: reclassification adjustment to net income, net of tax benefits of $37, $37, $112 and $112 for the respective periods	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 $1,900, $3,697, $5,438 and $11,173 for the respective periods	2,829	5,789	8,515	17,490
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, net of taxes of $1,619, $3,284, $4,858 and $9,852 for the respective periods	(2,542)	(5,156)	(7,627)	(15,468)
Other comprehensive income (loss), net of taxes	(1,311)	(897)	2,756	(9,263)
Comprehensive income	$ 46,504	$ 47,339	$ 137,919	$ 113,240