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Fair Value Measurements (Other Fair Value Disclosures) (Details) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Level 2 [Member] | Senior Unsecured Notes [Member] |  |
Other Fair Value Measurements |  |
Senior unsecured notes | $ 292.4 | $ 308.1

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