Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Change in net unrealized gains and losses, net of tax (provision) benefit of ($1.0), $1.0, and ($4.0) for 2019, 2018, and 2017, respectively	$ (1.0)	$ 1.0	$ (4.0)
Net realized (gains) losses reclassified into net income, net of tax provisions of $0.1, zero, and $0.9 for 2019, 2018, and 2017, respectively	0.1	0.0	0.9
Change in net unrealized gains and losses, net of tax (provision) benefit of ($2.6), $2.3, and ($4.4) for 2019, 2018, and 2017, respectively	(2.6)
Change in net unrealized gains and losses, net of tax (provision) benefit of ($2.6), $2.3, and ($4.4) for 2019, 2018, and 2017, respectively		2.3	(4.4)
Net realized losses (gains) reclassified into net income, net of tax provisions of $1.7, $0.3, and $2.4 for 2019, 2018, and 2017, respectively	$ 1.7
Net realized losses (gains) reclassified into net income, net of tax provisions of $1.7, $0.3, and $2.4 for 2019, 2018, and 2017, respectively		$ 0.3	$ 2.4