Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 151.0	$ 99.1	$ 271.1	$ 192.4
Other comprehensive income (loss), net of tax:
Unrealized gain (loss) on securities, net of tax	(0.1)	0.1	(0.4)	0.2
Other comprehensive income (loss)	(0.1)	0.1	(0.4)	0.2
Comprehensive income	$ 150.9	$ 99.2	$ 270.7	$ 192.6