Segment Information (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	$ 88,700,000		$ 88,700,000		$ 91,200,000
Operating revenues	2,578,913,000	2,274,668,000	5,361,762,000	4,852,747,000
Income (loss) from continuing operations	196,882,000	183,075,000	433,470,000	366,844,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	2,219,877,000	2,036,829,000	4,312,073,000	3,973,611,000
Income (loss) from continuing operations	201,084,000	190,151,000	375,190,000	333,372,000
Regulated Natural Gas
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	88,700,000		88,700,000		91,200,000
Operating revenues	341,321,000	221,313,000	1,010,917,000	842,348,000
Income (loss) from continuing operations	15,960,000	6,190,000	80,870,000	56,392,000
All Other
Segment Reporting Information [Line Items]
Operating revenues	17,715,000	16,526,000	38,772,000	36,788,000
Income (loss) from continuing operations	(20,162,000)	(13,266,000)	(22,590,000)	(22,920,000)
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	2,578,913,000	2,274,668,000	5,361,762,000	4,852,747,000
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	2,220,185,000	2,037,126,000	4,312,682,000	3,974,210,000
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	341,878,000	221,532,000	1,011,974,000	843,066,000
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	17,715,000	16,526,000	38,772,000	36,788,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(865,000)	(516,000)	(1,666,000)	(1,317,000)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	308,000	297,000	609,000	599,000
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	557,000	219,000	1,057,000	718,000
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0	$ 0	$ 0