Segment Information (Company's Segment Information, Continuing Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Operating revenues	$ 198,204	$ 178,313	$ 368,448	$ 341,928
Operations and maintenance expense	66,980	66,765	135,245	130,496
Depreciation	28,696	26,550	56,291	52,826
Operating income	88,395	72,623	151,474	133,543
Interest expense, net of AFUDC	18,294	17,612	36,198	34,991
Income tax expense (benefit)	27,787	18,554	45,746	31,570
Income (loss) from continuing operations	42,627	36,595	70,285	67,241
Capital expenditures			171,424	141,276
Regulated [Member]
Segment Reporting Information [Line Items]
Operating revenues	193,482	175,040	359,907	335,696
Operations and maintenance expense	64,225	63,784	129,547	125,920
Depreciation	29,014	27,065	56,928	53,682
Operating income	86,563	72,222	149,084	132,034
Interest expense, net of AFUDC	17,227	16,463	34,152	32,821
Income tax expense (benefit)	27,784	18,939	46,372	32,060
Income (loss) from continuing operations	41,616	36,843	68,810	67,251
Capital expenditures			171,046	140,399
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	4,722	3,273	8,541	6,232
Operations and maintenance expense	2,755	2,981	5,698	4,576
Depreciation	(318)	(515)	(637)	(856)
Operating income	1,832	401	2,390	1,509
Interest expense, net of AFUDC	1,067	1,149	2,046	2,170
Income tax expense (benefit)	3	(385)	(626)	(490)
Income (loss) from continuing operations	1,011	(248)	1,475	(10)
Capital expenditures			$ 378	$ 877