Consolidated Statements of Changes in Shareholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common stock	Retained earnings	Accumulated other comprehensive income (loss)	Comprehensive income
Balance at Dec. 31, 2008	$ 1,389,454	$ 1,231,629	$ 210,840	$ (53,015)
Balance (in shares) at Dec. 31, 2008		90,516,000
Increase (decrease) in stockholders' equity
Cumulative effect of adoption of a standard on other-than-temporary Impairment recognition, net of taxes of $2,497			3,781	(3,781)
Comprehensive income:
Net income for common stock	83,011		83,011		83,011
Net unrealized gains (losses) on securities:
Net unrealized gains (losses) on securities arising during the period, net of taxes of $4,343, $789 and $8,543 in 2011, 2010 and 2009, respectively	12,938			12,938	12,938
Less: reclassification adjustment for net realized gains (losses) included in net income, net of taxes of $148 and $18,882 in 2011 and 2009, respectively	28,596			28,596	28,596
Retirement benefit plans:
Net transition asset arising during the period, net of taxes of $4,172	6,549			6,549	6,549
Prior service credit arising during the period, net of taxes of $4,422, $3,001 and $921 in 2011, 2010 and 2009, respectively	1,446			1,446	1,446
Net gains (losses) arising during the period, net of tax benefits of $83,147, $28,431 and $41,218 in 2011, 2010 and 2009, respectively	64,547			64,547	64,547
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 $5,976, $2,566 and $6,861 in 2011, 2010 and 2009, respectively	10,754			10,754	10,754
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, net of tax benefits of $64,134, $21,336 and $48,251 in 2011, 2010 and 2009, respectively	(75,756)			(75,756)	(75,756)
Other comprehensive income (loss)				49,074
Comprehensive income	132,085				132,085
Issuance of common stock: Dividend reinvestment and stock purchase plan	27,701	27,701
Issuance of common stock: Dividend reinvestment and stock purchase plan (in shares)		1,714,000
Issuance of common stock: Retirement savings and other plans	4,771	4,771
Issuance of common stock: Retirement savings and other plans (in shares)		291,000
Issuance of common stock: Expenses and other, net	1,056	1,056
Common stock dividends ($1.24 per share)	(113,419)		(113,419)
Balance at Dec. 31, 2009	1,441,648	1,265,157	184,213	(7,722)
Balance (in shares) at Dec. 31, 2009		92,521,000
Comprehensive income:
Net income for common stock	113,535		113,535		113,535
Net unrealized gains (losses) on securities:
Net unrealized gains (losses) on securities arising during the period, net of taxes of $4,343, $789 and $8,543 in 2011, 2010 and 2009, respectively	(1,196)			(1,196)	(1,196)
Derivatives qualified as cash flow hedges:
Net unrealized holding losses arising during the period, net of tax benefits of $4 and $745 in 2011 and 2010, respectively	(1,169)			(1,169)	(1,169)
Retirement benefit plans:
Prior service credit arising during the period, net of taxes of $4,422, $3,001 and $921 in 2011, 2010 and 2009, respectively	4,712			4,712	4,712
Net gains (losses) arising during the period, net of tax benefits of $83,147, $28,431 and $41,218 in 2011, 2010 and 2009, respectively	(44,626)			(44,626)	(44,626)
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 $5,976, $2,566 and $6,861 in 2011, 2010 and 2009, respectively	4,030			4,030	4,030
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, net of tax benefits of $64,134, $21,336 and $48,251 in 2011, 2010 and 2009, respectively	33,499			33,499	33,499
Other comprehensive income (loss)				(4,750)
Comprehensive income	108,785				108,785
Issuance of common stock: Dividend reinvestment and stock purchase plan	37,296	37,296
Issuance of common stock: Dividend reinvestment and stock purchase plan (in shares)		1,685,000
Issuance of common stock: Retirement savings and other plans	8,934	8,934
Issuance of common stock: Retirement savings and other plans (in shares)		485,000
Issuance of common stock: Expenses and other, net	2,812	2,812
Common stock dividends ($1.24 per share)	(115,838)		(115,838)
Balance at Dec. 31, 2010	1,483,637	1,314,199	181,910	(12,472)
Balance (in shares) at Dec. 31, 2010	94,690,932	94,691,000
Comprehensive income:
Net income for common stock	138,230		138,230		138,230
Net unrealized gains (losses) on securities:
Net unrealized gains (losses) on securities arising during the period, net of taxes of $4,343, $789 and $8,543 in 2011, 2010 and 2009, respectively	6,578			6,578	6,578
Less: reclassification adjustment for net realized gains (losses) included in net income, net of taxes of $148 and $18,882 in 2011 and 2009, respectively	(224)			(224)	(224)
Derivatives qualified as cash flow hedges:
Net unrealized holding losses arising during the period, net of tax benefits of $4 and $745 in 2011 and 2010, respectively	(8)			(8)	(8)
Less: reclassification adjustment to net income, net of tax benefits of $115	181			181	181
Retirement benefit plans:
Prior service credit arising during the period, net of taxes of $4,422, $3,001 and $921 in 2011, 2010 and 2009, respectively	6,943			6,943	6,943
Net gains (losses) arising during the period, net of tax benefits of $83,147, $28,431 and $41,218 in 2011, 2010 and 2009, respectively	(130,191)			(130,191)	(130,191)
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 $5,976, $2,566 and $6,861 in 2011, 2010 and 2009, respectively	9,364			9,364	9,364
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, net of tax benefits of $64,134, $21,336 and $48,251 in 2011, 2010 and 2009, respectively	100,692			100,692	100,692
Other comprehensive income (loss)				(6,665)
Comprehensive income	131,565				131,565
Issuance of common stock: Dividend reinvestment and stock purchase plan	21,217	21,217
Issuance of common stock: Dividend reinvestment and stock purchase plan (in shares)		879,000
Issuance of common stock: Retirement savings and other plans	10,318	10,318
Issuance of common stock: Retirement savings and other plans (in shares)		468,000
Issuance of common stock: Expenses and other, net	3,712	3,712
Common stock dividends ($1.24 per share)	(118,500)		(118,500)
Balance at Dec. 31, 2011	$ 1,531,949	$ 1,349,446	$ 201,640	$ (19,137)
Balance (in shares) at Dec. 31, 2011	96,038,328	96,038,000