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Convertible Securities (Tables) | 12 Months Ended
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Dec. 31, 2017
Debt Disclosure [Abstract] |
Schedule of Carrying Value of Convertible Securities | The carrying value and principal amount at maturity of the junior convertible securities were as follows:
|  | December 31, 2016 |  | December 31, 2017
|  | Carrying Value |  | Principal Amount at Maturity |  | Carrying Value |  | Principal Amount at Maturity
Junior convertible securities(1) |  | $ | 307.5 |  |  | $ | 430.8 |  |  | $ | 309.9 |  |  | $ | 430.8 |
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(1) | The carrying value is accreted to the principal amount at maturity over a remaining life of 20 years.

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