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Condensed Parent Company Financial Statements (Condensed Balance Sheet) (Details) - USD ($) $ in Millions | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | [2]
--- | --- | --- | --- | --- | ---
Current assets: |  |  |  |  |  |  |
Other |  | $ 95.5 | $ 79.7 |  |  |  |
Total current assets |  | 877.1 | 826.8 |  |  |  |
Investments: |  |  |  |  |  |  |
Other |  | 20.0 | 53.0 |  |  |  |
Total investments |  | 337.6 | 346.3 |  |  |  |
Other assets |  | 22.6 | 14.6 |  |  |  |
Total assets |  | 13,373.8 | 12,495.2 |  | $ 12,063.5 |  |
Current liabilities: |  |  |  |  |  |  |
Current maturities of long-term debt |  | 4.6 | 313.4 |  |  |  |
Commercial paper |  | 244.1 | 159.8 |  |  |  |
Other |  | 222.3 | 243.4 |  |  |  |
Total current liabilities |  | 1,162.0 | 1,359.3 |  |  |  |
Other liabilities |  | 279.3 | 306.0 |  |  |  |
Common Equity: |  |  |  |  |  |  |
Retained earnings |  | 2,177.0 | 2,068.9 |  |  |  |
Shares in deferred compensation trust | [1] | (10.0) | (8.5) |  |  |  |
Total common equity |  | 3,862.0 | 3,724.1 | [2] | $ 3,438.7 | [2] | $ 3,281.4
Total liabilities and equity |  | 13,373.8 | 12,495.2 |  |  |  |
Parent Company [Member] |  |  |  |  |  |  |
Current assets: |  |  |  |  |  |  |
Notes receivable from affiliated companies |  | 74.0 | 93.0 |  |  |  |
Other |  | 5.0 | 9.0 |  |  |  |
Total current assets |  | 79.0 | 102.0 |  |  |  |
Investments: |  |  |  |  |  |  |
Investments in consolidated subsidiaries |  | 4,211.0 | 3,999.0 |  |  |  |
Other |  | 2.0 | 14.0 |  |  |  |
Total investments |  | 4,213.0 | 4,013.0 |  |  |  |
Other assets |  | 64.0 | 20.0 |  |  |  |
Total assets |  | 4,356.0 | 4,135.0 |  |  |  |
Current liabilities: |  |  |  |  |  |  |
Current maturities of long-term debt |  | 0.0 | 250.0 |  |  |  |
Commercial paper |  | 192.0 | 140.0 |  |  |  |
Notes payable to affiliated companies |  | 275.0 | 0.0 |  |  |  |
Other |  | 12.0 | 12.0 |  |  |  |
Total current liabilities |  | 479.0 | 402.0 |  |  |  |
Other liabilities |  | 18.0 | 12.0 |  |  |  |
Common Equity: |  |  |  |  |  |  |
Common stock and additional paid-in capital |  | 1,695.0 | 1,664.0 |  |  |  |
Retained earnings |  | 2,174.0 | 2,066.0 |  |  |  |
Shares in deferred compensation trust |  | (10.0) | (9.0) |  |  |  |
Total common equity |  | 3,859.0 | 3,721.0 |  |  |  |
Total liabilities and equity |  | $ 4,356.0 | $ 4,135.0 |  |  |  |
[1] | Amounts reflect the effects of a two-for-one common stock split distributed in May 2016.
[2] | Amounts reflect the effects of a two-for-one common stock split distributed in May 2016.

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