Consolidated Statements of Changes in Shareholders' Equity (USD $) In Thousands, except Share data	Total	Common stock	Retained earnings	Accumulated other comprehensive income (loss)	Comprehensive income
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 (loss):
Net income for common stock	88,837		88,837		88,837
Net unrealized gains on securities:
Net unrealized gains on securities arising during the period, net of taxes of $4,258 and $1,599 in September 2011 and September 2010, respectively	2,421			2,421	2,421
Derivatives qualified as cash flow hedges:
Net unrealized holding losses arising during the period, net of tax benefits of $4 and $2,278 in September 2011 and September 2010, respectively	(3,575)			(3,575)	(3,575)
Retirement benefit plans:
Less: amortization of net loss, prior service gain and transition obligation included in net periodic benefit cost, net of tax benefits of $3,513 and $1,932 in September 2011 and September 2010, respectively	3,034			3,034	3,034
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, net of taxes of $3,875 and $1,681 in September 2011 and September 2010, respectively	(2,640)			(2,640)	(2,640)
Other comprehensive income				(760)
Comprehensive income	88,077				88,077
Issuance of common stock, net	36,553	36,553
Issuance of common stock, net (in shares)		1,600,000
Common stock dividends ($0.93 per share)	(86,625)		(86,625)
Balance at Sep. 30, 2010	1,479,653	1,301,710	186,425	(8,482)
Balance (in shares) at Sep. 30, 2010		94,121,000
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 (loss):
Net income for common stock	104,005		104,005		104,005
Net unrealized gains on securities:
Net unrealized gains on securities arising during the period, net of taxes of $4,258 and $1,599 in September 2011 and September 2010, respectively	6,448			6,448	6,448
Less: reclassification adjustment for net realized gains included in net income, net of taxes of $148 in 2011	(224)			(224)	(224)
Derivatives qualified as cash flow hedges:
Net unrealized holding losses arising during the period, net of tax benefits of $4 and $2,278 in September 2011 and September 2010, respectively	(8)			(8)	(8)
Less: reclassification adjustment to net income, net of tax benefits of $78 in 2011	122			122	122
Retirement benefit plans:
Less: amortization of net loss, prior service gain and transition obligation included in net periodic benefit cost, net of tax benefits of $3,513 and $1,932 in September 2011 and September 2010, respectively	5,556			5,556	5,556
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, net of taxes of $3,875 and $1,681 in September 2011 and September 2010, respectively	(6,084)			(6,084)	(6,084)
Other comprehensive income				5,810
Comprehensive income	109,815				109,815
Issuance of common stock, net	33,056	33,056
Issuance of common stock, net (in shares)		1,284,000
Common stock dividends ($0.93 per share)	(88,750)		(88,750)
Balance at Sep. 30, 2011	$ 1,537,758	$ 1,347,255	$ 197,165	$ (6,662)
Balance (in shares) at Sep. 30, 2011	95,975,524	95,975,000