Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 85.9	$ 74.0	$ 245.2	$ 219.7
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	$ 85.8	$ 74.1	$ 245.6	$ 219.9