Consolidated Statements Of Comprehensive Income (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax [Abstract]
Net income	$ 266.2	$ 253.1	$ 219.7
Unrealized gain (loss) on securities, net of tax	0.1	(0.1)	(0.1)
Other comprehensive income (loss)	0.1	(0.1)	(0.1)
Comprehensive income	$ 266.3	$ 253.0	$ 219.6