Segment Information (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	$ 85,500,000		$ 85,500,000		$ 87,100,000
Operating revenues	2,685,096,000	2,578,913,000	5,887,700,000	5,361,762,000
Net income (loss)	195,164,000	196,857,000	456,385,000	433,427,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	2,298,075,000	2,220,185,000	4,600,138,000	4,312,682,000
Net income (loss)	185,677,000	201,084,000	371,110,000	375,190,000
Regulated Natural Gas
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	85,500,000		85,500,000		87,100,000
Operating revenues	370,245,000	341,878,000	1,253,185,000	1,011,974,000
Net income (loss)	15,297,000	15,960,000	92,633,000	80,870,000
All Other
Segment Reporting Information [Line Items]
Operating revenues	18,331,000	17,715,000	39,537,000	38,772,000
Net income (loss)	(5,810,000)	(20,187,000)	(7,358,000)	(22,633,000)
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	2,685,096,000	2,578,913,000	5,887,700,000	5,361,762,000
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	2,297,638,000	2,219,877,000	4,599,348,000	4,312,073,000
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	369,127,000	341,321,000	1,248,815,000	1,010,917,000
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	18,331,000	17,715,000	39,537,000	38,772,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(1,555,000)	(865,000)	(5,160,000)	(1,666,000)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	437,000	308,000	790,000	609,000
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	1,118,000	557,000	4,370,000	1,057,000
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0	$ 0	$ 0