Segment Information (Company's Segment Information, Continuing Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Operating revenues	$ 210,535	$ 202,320	$ 588,514	$ 574,815
Operations and maintenance expense	72,374	71,631	214,435	209,837
Depreciation	29,482	29,980	91,689	88,336
Operating income (loss)	95,058	86,140	241,762	232,828
Interest expense, net of AFUDC	17,795	18,924	54,094	56,365
Income tax expense (benefit)	8,891	4,683	19,932	18,235
Income (loss) from continuing operations	67,711	62,749	164,930	156,617
Capital expenditures			220,739	215,407
Regulated Assets [Member]
Segment Reporting Information [Line Items]
Operating revenues	203,303	197,857	571,180	562,035
Operations and maintenance expense	68,319	67,737	204,291	200,641
Depreciation	29,273	29,873	91,378	88,565
Operating income (loss)	92,783	86,232	236,638	231,032
Interest expense, net of AFUDC	16,805	17,314	50,654	51,334
Income tax expense (benefit)	8,832	5,435	20,701	20,483
Income (loss) from continuing operations	67,221	63,566	165,587	159,380
Capital expenditures			219,465	214,575
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	7,232	4,463	17,334	12,780
Operations and maintenance expense	4,055	3,894	10,144	9,196
Depreciation	209	107	311	(229)
Operating income (loss)	2,275	(92)	5,124	1,796
Interest expense, net of AFUDC	990	1,610	3,440	5,031
Income tax expense (benefit)	59	(752)	(769)	(2,248)
Income (loss) from continuing operations	490	(817)	(657)	(2,763)
Capital expenditures			$ 1,274	$ 832