Segment Information (Company's Segment Information, Revenues) (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Operating revenues	$ 197,328	$ 193,477	$ 539,256	$ 515,003
Operations and maintenance expense	70,039	68,908	200,535	198,448
Depreciation	27,132	26,494	79,884	77,635
Operating income	87,759	83,323	221,376	197,515
Interest expense, net of AFUDC	17,756	17,556	52,747	50,523
Gain on sale of other assets	216	291	475	2,294
Income tax	24,703	25,728	56,303	58,573
Income from continuing operations	45,516	40,330	112,801	90,713
Capital expenditures			226,075	231,542
Regulated [Member]
Segment Reporting Information [Line Items]
Operating revenues	194,068	190,656	529,764	506,820
Operations and maintenance expense	69,016	66,014	196,033	191,224
Depreciation	27,461	26,706	81,069	78,629
Operating income	85,573	83,436	216,584	196,758
Interest expense, net of AFUDC	16,668	16,210	49,489	47,810
Gain on sale of other assets			178	221
Income tax	24,396	26,798	55,985	59,996
Income from continuing operations	44,589	40,692	111,288	89,173
Capital expenditures			225,007	231,216
Other Reporting Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	3,260	2,821	9,492	8,183
Operations and maintenance expense	1,023	2,894	4,502	7,224
Depreciation	(329)	(212)	(1,185)	(994)
Operating income	2,186	(113)	4,792	757
Interest expense, net of AFUDC	1,088	1,346	3,258	2,713
Gain on sale of other assets			297	2,073
Income tax	307	(1,070)	318	(1,423)
Income from continuing operations	927	(362)	1,513	1,540
Capital expenditures			$ 1,068	$ 326