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Investments - Schedule of Available-for-sale Securities (Parenthetical) (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Asset-backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Other-than-temporary impairment losses, investments, portion in other comprehensive income (loss) $ 0.2 $ 0.2