Statements Of Consolidated and Combined Cash Flows (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Operating Activities    
Net Income $ 131.2  
Adjustments to Reconcile Net Income to Net Cash from Continuing Operations:    
Depreciation and amortization 32.3  
Deferred income taxes and investment tax credits 10.5  
Deferred revenue 5.3  
Equity-based compensation expense and 401(k) profit sharing contribution 2.0  
Gain on sale of assets (5.3)  
Income from unconsolidated affiliates (14.9)  
Amortization of Financing Costs 0.1  
AFUDC equity (3.5)  
Distributions of earnings received from equity investees 18.3  
Changes in Assets and Liabilities:    
Accounts receivable 12.2  
Accounts receivable-affiliated 15.1  
Accounts payable (15.6)  
Accounts payable-affiliated (15.1)  
Customer deposits 0.6  
Taxes accrued 2.4  
Exchange gas receivable/payable 0  
Other accruals (7.8)  
Prepayments and other current assets 2.9  
Regulatory assets/liabilities 11.8  
Postretirement and postemployment benefits (7.7)  
Deferred charges and other noncurrent assets (1.9)  
Other noncurrent liabilities 0.8  
Net Cash Flows from Operating Activities 173.7  
Investing Activities    
Capital expenditures (163.9)  
Changes in short-term lendings-affiliated (699.6)  
Proceeds from disposition of assets 10.2  
Distributions from or contributions to equity investees 1.3  
Other investing activities (2.5)  
Net Cash Flows used for Investing Activities (854.5)  
Financing Activities    
Change in short-term borrowings-affiliated (240.4)  
Payments of long-term debt-affiliated, including current portion (957.8)  
Proceeds from the issuance of common units, net of offering costs 1,168.4  
Distribution of IPO proceeds to parent (500.0)  
Contribution of capital from parent 1,217.3  
Net Cash Flows from (used for) Financing Activities 687.5  
Change in cash and cash equivalents 6.7  
Cash and cash equivalents at beginning of period 0.5  
Cash and Cash Equivalents at End of Period 7.2  
Predecessor [Member]    
Operating Activities    
Net Income   92.5
Adjustments to Reconcile Net Income to Net Cash from Continuing Operations:    
Depreciation and amortization   29.7
Deferred income taxes and investment tax credits   28.9
Deferred revenue   0.8
Equity-based compensation expense and 401(k) profit sharing contribution   0.9
Gain on sale of assets   (17.5)
Income from unconsolidated affiliates   (9.8)
Amortization of Financing Costs   0
AFUDC equity   (1.6)
Distributions of earnings received from equity investees   7.6
Changes in Assets and Liabilities:    
Accounts receivable   (5.2)
Accounts receivable-affiliated   16.1
Accounts payable   (2.8)
Accounts payable-affiliated   (10.1)
Customer deposits   75.8
Taxes accrued   (5.3)
Exchange gas receivable/payable   (0.2)
Other accruals   (4.7)
Prepayments and other current assets   3.1
Regulatory assets/liabilities   11.3
Postretirement and postemployment benefits   (7.1)
Deferred charges and other noncurrent assets   (0.3)
Other noncurrent liabilities   0.4
Net Cash Flows from Operating Activities   202.5
Investing Activities    
Capital expenditures   (133.4)
Changes in short-term lendings-affiliated   (1.3)
Proceeds from disposition of assets   4.9
Distributions from or contributions to equity investees   (31.0)
Other investing activities   (1.9)
Net Cash Flows used for Investing Activities   (162.7)
Financing Activities    
Change in short-term borrowings-affiliated   (39.9)
Payments of long-term debt-affiliated, including current portion   0
Proceeds from the issuance of common units, net of offering costs   0
Distribution of IPO proceeds to parent   0
Contribution of capital from parent   0
Net Cash Flows from (used for) Financing Activities   (39.9)
Change in cash and cash equivalents   (0.1)
Cash and cash equivalents at beginning of period   0.3
Cash and Cash Equivalents at End of Period   $ 0.2