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Affiliate Equity - Effect of Changes in the Company's Ownership Interest in its Affiliates on the Controlling Interest's Equity (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
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Dec. 31, 2017 | [1] | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Affiliate Equity [Abstract] |  |  |  |  |  |  |  |  |  |  |
Net income (controlling interest) | $ 315.4 | $ 125.4 | $ 126.3 | $ 122.5 | $ 150.2 | $ 110.2 | $ 108.3 | $ 104.0 | $ 689.5 | $ 472.8 | $ 509.5
Increase / (decrease) in controlling interest paid-in capital from purchases and sales of Affiliate equity issuances |  |  |  |  |  |  |  |  | (1.0) | 1.6 | 0.9
Decrease in controlling interest paid-in capital related to Affiliate equity repurchases |  |  |  |  |  |  |  |  | (116.2) | (38.0) | (87.6)
Net income attributable to controlling interest and transfers from non-controlling interests |  |  |  |  |  |  |  |  | $ 572.3 | $ 436.4 | $ 422.8
[1] | In the fourth quarter of 2017, the Company recorded a provisional one-time net benefit from changes in U.S tax laws (see Note 22) and an expense associated with the impairment of one of its Affiliates accounted for under the equity method (see Note 13).

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- Definition Adjustments to Additional Paid in Capital Decrease in Carrying Amount of Redeemable Noncontrolling Interests + References No definition available. + Details
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