Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 151.0	$ 100.7	$ 422.1	$ 293.1
Other comprehensive income (loss), net of tax:
Unrealized gain (loss) on securities, net of tax	0.0	0.0	(0.4)	0.2
Other comprehensive income (loss)	0.0	0.0	(0.4)	0.2
Comprehensive income	$ 151.0	$ 100.7	$ 421.7	$ 293.3