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Condensed Parent Company Financial Statements (Condensed Statements of Cash Flow) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Net cash flows from operating activities | $ 859.6 | $ 871.2 |  | $ 891.6
Cash flows from (used for) investing activities: |  |  |  |
Other | 10.3 | (24.8) |  | (14.9)
Net cash flows from (used for) investing activities | (1,186.5) | (919.2) |  | (917.7)
Cash flows used for financing activities: |  |  |  |
Common stock dividends | (266.5) | (247.3) |  | (225.8)
Proceeds from issuance of common stock, net | 26.6 | 151.2 | [1] | 0.0
Proceeds from issuance of long-term debt | 800.0 | 250.7 |  | 812.9
Payments to retire long-term debt | (313.4) | (183.0) |  | (358.5)
Net change in commercial paper | 84.3 | 18.5 |  | (138.1)
Other | (1.7) | 6.8 |  | (17.3)
Net cash flows from (used for) financing activities | 329.3 | (3.1) |  | 73.2
Net increase (decrease) in cash and cash equivalents | 2.4 | (51.1) |  | 47.1
Cash and cash equivalents at beginning of period | 5.8 | 56.9 |  | 9.8
Cash and cash equivalents at end of period | 8.2 | 5.8 |  | 56.9
Supplemental cash flows information: |  |  |  |
Interest, net of capitalized interest | (192.4) | (184.8) |  | (180.8)
Income taxes, net | (9.8) | 0.0 |  | 5.3
Parent Company [Member] |  |  |  |
Net cash flows from operating activities | 254.0 | 262.0 |  | 246.0
Cash flows from (used for) investing activities: |  |  |  |
Capital contributions to consolidated subsidiaries | (250.0) | (165.0) |  | (90.0)
Captial repayments from consolidated subsidiaries | 130.0 | 0.0 |  | 50.0
Net change in notes receivable from and payable to affiliates | 294.0 | 2.0 |  | (23.0)
Other | 10.0 | 0.0 |  | 0.0
Net cash flows from (used for) investing activities | 184.0 | (163.0) |  | (63.0)
Cash flows used for financing activities: |  |  |  |
Common stock dividends | (267.0) | (247.0) |  | (226.0)
Proceeds from issuance of common stock, net | 27.0 | 151.0 |  | 0.0
Proceeds from issuance of long-term debt | 0.0 | 0.0 |  | 250.0
Payments to retire long-term debt | (250.0) | 0.0 |  | (250.0)
Net change in commercial paper | 52.0 | (1.0) |  | 45.0
Other | 0.0 | (2.0) |  | (2.0)
Net cash flows from (used for) financing activities | (438.0) | (99.0) |  | (183.0)
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) |  | (11.0)
Income taxes, net | $ (37.0) | $ (9.0) |  | $ (5.0)
[1] | Amounts reflect the effects of a two-for-one common stock split distributed in May 2016.

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