Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 123.4	$ 80.2	$ 341.8	$ 213.5
Other comprehensive income (loss):
Change in unrealized gains (losses) on investments	(1.4)	0.2	0.7	2.7
Less: tax provision (benefit) related to change in unrealized gains (losses) on investments	(0.3)	0.0	0.4	0.7
Other comprehensive income (loss)	(1.1)	0.2	0.3	2.0
Comprehensive income	$ 122.3	$ 80.4	$ 342.1	$ 215.5