Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 120.1	$ 93.3
Other comprehensive income (loss), net of tax:
Unrealized gain (loss) on securities, net of tax	(0.3)	0.1
Other comprehensive income (loss)	(0.3)	0.1
Comprehensive income	$ 119.8	$ 93.4