Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net change in unrealized gains on securities, taxes	$ 16.5	$ (12.9)	$ 42.6
Net change in other-than-temporary impairment gains (losses) recognized in other comprehensive income, taxes	$ 0.6	$ (0.3)	$ 0.4

Amount of tax (expense) benefit of other than temporary impairment (OTTI) on investment in available-for-sale debt security, recognized in other comprehensive loss, attributable to the parent entity.

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