Consolidated Statements Of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax [Abstract]
Net income	$ 470.2	$ 332.8	$ 299.7
Unrealized gain (loss) on securities, net of tax	0.0	(0.1)	0.0
Other comprehensive income (loss)	0.0	(0.1)	0.0
Comprehensive income	$ 470.2	$ 332.7	$ 299.7