Segments and Related Information (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Intercompany revenue									$ 14,000,000		$ 13,000,000		$ 14,000,000
Operating revenues	1,136,791,000	1,049,111,000	993,704,000	1,203,543,000	1,074,442,000	1,044,300,000	966,429,000	1,117,457,000	4,383,149,000		4,202,628,000		3,969,293,000
Depreciation and amortization									379,202,000		360,417,000		338,827,000
Total interest charges and financing costs									154,640,000		160,945,000		179,689,000
Income tax expense (benefit)									243,591,000		250,740,000		232,544,000
Net income (loss)	92,834,000	154,159,000	89,792,000	118,403,000	73,213,000	166,266,000	97,299,000	116,605,000	455,188,000		453,383,000		458,076,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									3,126,276,000		3,081,473,000		2,970,181,000
Depreciation and amortization									285,968,000		280,972,000		267,944,000
Total interest charges and financing costs									124,118,000		129,787,000		145,641,000
Income tax expense (benefit)									208,095,000		220,356,000		212,347,000
Net income (loss)									349,793,000		368,586,000		387,724,000
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									1,215,504,000		1,080,813,000		962,525,000
Depreciation and amortization									89,186,000		75,510,000		66,983,000
Total interest charges and financing costs									29,987,000		30,604,000		33,430,000
Income tax expense (benefit)									50,874,000		42,294,000		27,968,000
Net income (loss)									84,324,000		69,682,000		60,003,000
All Other
Segment Reporting Information [Line Items]
Operating revenues									41,888,000		40,754,000		36,959,000
Depreciation and amortization									4,048,000		3,935,000		3,900,000
Total interest charges and financing costs									535,000		554,000		618,000
Income tax expense (benefit)									(15,378,000)		(11,910,000)		(7,771,000)
Net income (loss)									21,071,000		15,115,000		10,349,000
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues									4,383,149,000	[1]	4,202,628,000	[1]	3,969,293,000	[1]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									3,125,937,000	[1]	3,081,171,000	[1]	2,969,899,000	[1]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									1,215,324,000	[1]	1,080,703,000	[1]	962,435,000	[1]
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues									41,888,000	[1]	40,754,000	[1]	36,959,000	[1]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues									(519,000)		(412,000)		(372,000)
Depreciation and amortization									0		0		0
Total interest charges and financing costs									0		0		0
Income tax expense (benefit)									0		0		0
Net income (loss)									0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									339,000		302,000		282,000
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									180,000		110,000		90,000
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues									$ 0		$ 0		$ 0

[1] Operating revenues include $14 million, $13 million and $14 million of intercompany revenue for the years ended Dec. 31, 2014, 2013 and 2012, respectively. See Note 16 for further discussion of related party transactions by reportable segment.