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Condensed Parent Company Financial Statements (Condensed Statements of Cash Flow) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Net cash flows from operating activities | $ 983.4 | $ 859.6 | $ 871.2
Cash flows from (used for) investing activities: |  |  |
Other | (29.4) | 10.3 | (24.8)
Net cash flows from (used for) investing activities | (1,496.3) | (1,186.5) | (919.2)
Cash flows used for financing activities: |  |  |
Common stock dividends | (288.3) | (266.5) | (247.3)
Proceeds from issuance of common stock, net | 149.6 | 26.6 | 151.2
Payments to retire long-term debt | (4.6) | (313.4) | (183.0)
Net change in commercial paper | 171.1 | 84.3 | 18.5
Other | (45.2) | (1.7) | 6.8
Net cash flows from (used for) financing activities | 532.6 | 329.3 | (3.1)
Net increase (decrease) in cash and cash equivalents | 19.7 | 2.4 | (51.1)
Cash and cash equivalents at beginning of period | 8.2 | 5.8 | 56.9
Cash and cash equivalents at end of period | 27.9 | 8.2 | 5.8
Supplemental cash flows information: |  |  |
Interest, net of capitalized interest | (212.6) | (192.4) | (184.8)
Income taxes, net | (11.3) | (9.8) | 0.0
Parent Company [Member] |  |  |
Net cash flows from operating activities | 273.0 | 254.0 | 262.0
Cash flows from (used for) investing activities: |  |  |
Capital contributions to consolidated subsidiaries | (290.0) | (250.0) | (165.0)
Captial repayments from consolidated subsidiaries | 0.0 | 130.0 | 0.0
Net change in notes receivable from and payable to affiliates | 54.0 | 294.0 | 2.0
Other | 0.0 | 10.0 | 0.0
Net cash flows from (used for) investing activities | (236.0) | 184.0 | (163.0)
Cash flows used for financing activities: |  |  |
Common stock dividends | (288.0) | (267.0) | (247.0)
Proceeds from issuance of common stock, net | 150.0 | 27.0 | 151.0
Payments to retire long-term debt | 0.0 | (250.0) | 0.0
Net change in commercial paper | 103.0 | 52.0 | (1.0)
Other | (2.0) | 0.0 | (2.0)
Net cash flows from (used for) financing activities | (37.0) | (438.0) | (99.0)
Net increase (decrease) in cash and cash equivalents | 0.0 | 0.0 | 0.0
Cash and cash equivalents at beginning of period | 0.0 | 0.0 | 0.0
Cash and cash equivalents at end of period | 0.0 | 0.0 | 0.0
Supplemental cash flows information: |  |  |
Interest, net of capitalized interest | (3.0) | (3.0) | (3.0)
Income taxes, net | $ 0.0 | $ (37.0) | $ (9.0)

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