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Investments - Net Effect on Stockholders' Equity of Unrealized Gains and Losses on Available-for-sale Securities (Parenthetical) (Details) $ in Millions | Dec. 31, 2017 USD ($)
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Prior to adoption of ASU 2016-01 [Member] |
Schedule Of Trading Securities And Other Trading Assets [Line Items] |
Net unrealized gains for equity securities recognized in accumulated other comprehensive income (loss) prior to adoption of ASU 2016-01 | $ 9.8

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