Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Change in net unrealized gains and losses, net of tax (provision) benefit of $(1.9), $(1.0), and $1.0 for 2020, 2019, and 2018, respectively	$ (1.9)	$ (1.0)	$ 1.0
Change in net unrealized gains and losses, net of tax (provision) benefit of $(9.1), $(2.6), and $2.3 for 2020, 2019, and 2018, respectively	(9.1)	(2.6)	2.3
Net realized losses (gains) reclassified into net income, net of tax benefit (provisions) of $1.4, $(1.7), and $(0.3) for 2020, 2019, and 2018, respectively	$ (1.4)	$ 1.7	$ 0.3