Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 165.4	$ 151.0	$ 494.2	$ 422.1
Other comprehensive income (loss), net of tax:
Unrealized gain (loss) on securities, net of tax	0.1	0.0	1.2	(0.4)
Other comprehensive income (loss)	0.1	0.0	1.2	(0.4)
Comprehensive income	$ 165.5	$ 151.0	$ 495.4	$ 421.7