Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 74.0	$ 74.0	$ 219.7	$ 193.2
Other comprehensive income (loss), net of tax:
Unrealized gain (loss) on securities, net of tax	0.1	(0.1)	0.2	0.1
Other comprehensive income (loss)	0.1	(0.1)	0.2	0.1
Comprehensive income	$ 74.1	$ 73.9	$ 219.9	$ 193.3