Consolidated Statements Of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax [Abstract]
Net income	$ 299.7	$ 266.2	$ 253.1
Unrealized gain (loss) on securities, net of tax	0.0	0.1	(0.1)
Other comprehensive income (loss)	0.0	0.1	(0.1)
Comprehensive income	$ 299.7	$ 266.3	$ 253.0