Consolidated Statements Of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax [Abstract]
Net income	$ 574.0	$ 470.2	$ 332.8
Unrealized gain (loss) on securities, net of tax	(0.1)	0.0	(0.1)
Other comprehensive income (loss)	(0.1)	0.0	(0.1)
Comprehensive income	$ 573.9	$ 470.2	$ 332.7