Segments and Related Information (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Segment Reporting Information [Line Items]
Intercompany revenue									$ 12,600,000		$ 13,700,000		$ 15,300,000
Operating revenues	1,074,442,000	1,044,300,000	966,429,000	1,117,457,000	1,031,453,000	992,287,000	869,501,000	1,076,052,000	4,202,628,000		3,969,293,000		4,240,802,000
Depreciation and amortization									360,417,000		338,827,000		328,582,000
Total interest charges and financing costs									160,945,000		179,689,000		183,479,000
Income Tax Expense (Benefit)									250,740,000		232,544,000		228,361,000
Net income	73,213,000	166,266,000	97,299,000	116,605,000	76,941,000	192,443,000	95,407,000	93,285,000	453,383,000		458,076,000		396,803,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									3,081,473,000		2,970,181,000		3,114,688,000
Depreciation and amortization									280,972,000		267,944,000		265,078,000
Total interest charges and financing costs									129,787,000		145,641,000		149,291,000
Income Tax Expense (Benefit)									220,356,000		212,347,000		202,355,000
Net income									368,586,000		387,724,000		334,516,000
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									1,080,813,000		962,525,000		1,087,991,000
Depreciation and amortization									75,510,000		66,983,000		59,189,000
Total interest charges and financing costs									30,604,000		33,430,000		33,249,000
Income Tax Expense (Benefit)									42,294,000		27,968,000		30,957,000
Net income									69,682,000		60,003,000		55,446,000
All Other
Segment Reporting Information [Line Items]
Operating revenues									40,754,000		36,959,000		38,683,000
Depreciation and amortization									3,935,000		3,900,000		4,315,000
Total interest charges and financing costs									554,000		618,000		939,000
Income Tax Expense (Benefit)									(11,910,000)		(7,771,000)		(4,951,000)
Net income									15,115,000		10,349,000		6,841,000
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues									4,202,628,000	[1]	3,969,293,000	[1]	4,240,802,000	[1]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									3,081,171,000	[1]	2,969,899,000	[1]	3,114,370,000	[1]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									1,080,703,000	[1]	962,435,000	[1]	1,087,749,000	[1]
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues									40,754,000	[1]	36,959,000	[1]	38,683,000	[1]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues									(412,000)		(372,000)		(560,000)
Depreciation and amortization									0		0		0
Total interest charges and financing costs									0		0		0
Income Tax Expense (Benefit)									0		0		0
Net income									0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									302,000		282,000		318,000
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									110,000		90,000		242,000
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues									$ 0		$ 0		$ 0

[1] Operating revenues include $12.6 million, $13.7 million and $15.3 million of intercompany revenue for the years ended Dec. 31, 2013, 2012 and 2011, respectively. See Note 16 for further discussion of related party transactions by reportable segment.