Consolidated Statements of Comprehensive Income (Unaudited) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 71.5	$ 49.8
Other comprehensive income, net of tax:
Unrealized gain (loss) on securities, net of tax	0.3	0.1
Other comprehensive income	0.3	0.1
Comprehensive income	$ 71.8	$ 49.9