Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 100.7	$ 85.9	$ 293.1	$ 245.2
Other comprehensive income (loss), net of tax:
Unrealized gain (loss) on securities, net of tax	0.0	(0.1)	0.2	0.4
Other comprehensive income (loss)	0.0	(0.1)	0.2	0.4
Comprehensive income	$ 100.7	$ 85.8	$ 293.3	$ 245.6