Segment Information (Summarized Segment Information) (Details) (USD $)
In Thousands
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Dec. 31, 2010
Segment Reporting Information [Line Items]     
Net operating revenues$ 766,450$ 748,953$ 2,042,520$ 1,941,883 
Depreciation and amortization88,42484,005263,027246,759 
Total operating expenses, net485,133482,4251,416,5241,363,476 
Income (loss) from continuing operations before income taxes206,759195,119401,457357,203 
Total assets14,538,73313,983,675[1]14,538,73313,983,675[1]14,086,246
Capital expenditures230,148194,811621,940522,090 
Assets of discontinued operations related to segment information943,155926,855943,155926,855 
Capital expenditures from discontinued operations  15,61917,096 
Regulated Unit [Member]
     
Segment Reporting Information [Line Items]     
Net operating revenues682,363675,0871,805,0851,730,544 
Depreciation and amortization81,05876,381239,435224,609 
Total operating expenses, net417,020417,5611,212,0861,174,728 
Income (loss) from continuing operations before income taxes209,722203,230424,804391,207 
Total assets12,669,871[1]12,201,526[1]12,669,871[1]12,201,526[1] 
Capital expenditures229,120[2]193,540[2]619,119[2]516,598[2] 
Market-Based Operations [Member]
     
Segment Reporting Information [Line Items]     
Net operating revenues91,62880,310259,916230,153 
Depreciation and amortization1,7661,8545,6455,562 
Total operating expenses, net77,54871,499232,023214,285 
Income (loss) from continuing operations before income taxes14,6829,43129,58519,679 
Total assets267,905[1]244,394[1]267,905[1]244,394[1] 
Capital expenditures1,046[2]1,271[2]2,821[2]5,492[2] 
Other [Member]
     
Segment Reporting Information [Line Items]     
Net operating revenues(7,541)(6,444)(22,481)(18,814) 
Depreciation and amortization5,6005,77017,94716,588 
Total operating expenses, net(9,435)(6,635)(27,585)(25,537) 
Income (loss) from continuing operations before income taxes(17,645)(17,542)(52,932)(53,683) 
Total assets1,600,957[1]1,537,755[1]1,600,957[1]1,537,755[1] 
Capital expenditures$ 0[2]$ 0[2]$ 0[2]$ 0[2] 
[1]The Regulated Businesses segment includes $943,155 and $926,855 of assets of discontinued operations as of September 30, 2011 and 2010, respectively.
[2]The Regulated Businesses segment includes capital expenditures from discontinued operations totaling $15,619 and $17,096 for the nine months and $4,600 and $4,714 for the three months ended September 30, 2011 and 2010, respectively.