CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) shares in Millions, $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Statement Of Financial Position [Abstract]
Property and equipment, accumulated depreciation	$ 337.0	$ 312.0
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	1.0	1.0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	200.0	200.0
Common stock, shares issued	106.9	106.7
Treasury stock	1.9	0.7