Consolidated Statements of Changes in Shareholders' Equity (USD $) In Thousands	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
Comprehensive income (loss):
Net income for common stock	56,388		56,388		56,388
Net unrealized gains on securities:
Net unrealized gains on securities arising during the period, net of taxes of $2,341 and $1,747 in June 2011 and June 2010, respectively	2,646			2,646	2,646
Derivatives qualified as cash flow hedges:
Net unrealized holding losses arising during the period, net of tax benefits of $9 and $662 in June 2011 and June 2010, respectively	(1,039)			(1,039)	(1,039)
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 $2,108 and $1,248 in June 2011 and June 2010, respectively	1,959			1,959	1,959
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, net of taxes of $2,801 and $1,080 in June 2011 and June 2010, respectively	(1,697)			(1,697)	(1,697)
Other comprehensive income				1,869
Comprehensive income	58,257				58,257
Issuance of common stock, net	24,314	24,314
Issuance of common stock, net (in shares)		1,099
Common stock dividends ($0.62 per share)	(57,586)		(57,586)
Balance at Jun. 30, 2010	1,466,633	1,289,471	183,015	(5,853)
Balance (in shares) at Jun. 30, 2010		93,620
Balance at Dec. 31, 2010	1,483,637	1,314,199	181,910	(12,472)
Balance (in shares) at Dec. 31, 2010		94,691
Comprehensive income (loss):
Net income for common stock	55,601		55,601		55,601
Net unrealized gains on securities:
Net unrealized gains on securities arising during the period, net of taxes of $2,341 and $1,747 in June 2011 and June 2010, respectively	3,435			3,435	3,435
Less: reclassification adjustment for net realized gains included in net income, net of taxes of $2 in 2011	(3)			(3)	(3)
Derivatives qualified as cash flow hedges:
Net unrealized holding losses arising during the period, net of tax benefits of $9 and $662 in June 2011 and June 2010, respectively	(3)			(3)	(3)
Less: reclassification adjustment to net income, net of tax benefits of $41 in 2011	64			64	64
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 $2,108 and $1,248 in June 2011 and June 2010, respectively	3,488			3,488	3,488
Less: reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, net of taxes of $2,801 and $1,080 in June 2011 and June 2010, respectively	(4,352)			(4,352)	(4,352)
Other comprehensive income				2,629
Comprehensive income	58,230				58,230
Issuance of common stock, net	29,338	29,338
Issuance of common stock, net (in shares)		1,162
Common stock dividends ($0.62 per share)	(58,998)		(58,998)
Balance at Jun. 30, 2011	$ 1,512,207	$ 1,343,537	$ 178,513	$ (9,843)
Balance (in shares) at Jun. 30, 2011		95,853