Consolidated Statements Of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax [Abstract]
Net income	$ 332.8	$ 299.7	$ 266.2
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	$ 332.7	$ 299.7	$ 266.3