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Condensed Parent Company Financial Statements (Condensed Balance Sheet) (Details) - USD ($) $ in Millions | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
--- | --- | --- | --- | ---
Current assets: |  |  |  |
Other | $ 79.7 | $ 116.9 |  |
Total current assets | 826.8 | 1,043.1 |  |
Investments: |  |  |  |
Other | 53.0 | 58.4 |  |
Total investments | 346.3 | 344.9 |  |
Other assets | 1,803.0 | 1,737.1 |  |
Total assets | 12,495.2 | 12,063.5 | $ 11,092.5 |
Current liabilities: |  |  |  |
Current maturities of long-term debt | 313.4 | 183.0 |  |
Commercial paper | 159.8 | 141.3 |  |
Other | 296.6 | 262.4 |  |
Total current liabilities | 1,359.3 | 1,214.7 |  |
Long-term debt, net (excluding current portion) | 3,522.2 | 3,584.3 |  |
Other liabilities | 306.0 | 260.1 |  |
Common Equity: |  |  |  |
Retained earnings | 2,068.9 | 1,938.0 |  |
Shares in deferred compensation trust | (8.5) | (8.9) |  |
Total common equity | 3,724.1 | 3,438.7 | $ 3,281.4 | $ 3,134.9
Total liabilities and equity | 12,495.2 | 12,063.5 |  |
Parent Company [Member] |  |  |  |
Current assets: |  |  |  |
Notes receivable from affiliated companies | 93.0 | 95.0 |  |
Other | 9.0 | 25.0 |  |
Total current assets | 102.0 | 120.0 |  |
Investments: |  |  |  |
Investments in consolidated subsidiaries | 3,999.0 | 3,721.0 |  |
Other | 14.0 | 15.0 |  |
Total investments | 4,013.0 | 3,736.0 |  |
Other assets | 20.0 | 10.0 |  |
Total assets | 4,135.0 | 3,866.0 |  |
Current liabilities: |  |  |  |
Current maturities of long-term debt | 250.0 | 0.0 |  |
Commercial paper | 140.0 | 141.0 |  |
Other | 12.0 | 29.0 |  |
Total current liabilities | 402.0 | 170.0 |  |
Long-term debt, net (excluding current portion) | 0.0 | 250.0 |  |
Other liabilities | 12.0 | 11.0 |  |
Common Equity: |  |  |  |
Common stock and additional paid-in capital | 1,664.0 | 1,510.0 |  |
Retained earnings | 2,066.0 | 1,934.0 |  |
Shares in deferred compensation trust | (9.0) | (9.0) |  |
Total common equity | 3,721.0 | 3,435.0 |  |
Total liabilities and equity | $ 4,135.0 | $ 3,866.0 |  |

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