Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 164.4	$ 151.0	$ 328.8	$ 271.1
Other comprehensive income (loss), net of tax:
Unrealized gain (loss) on securities, net of tax	0.5	(0.1)	1.1	(0.4)
Other comprehensive income (loss)	0.5	(0.1)	1.1	(0.4)
Comprehensive income	$ 164.9	$ 150.9	$ 329.9	$ 270.7