Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Millions, $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts	$ 32.1	$ 24.9
Accumulated depreciation	1,279.2	1,233.4
Intangible assets, accumulated amortization	$ 879.9	$ 777.6
Preferred stock, par value	$ 1.00	$ 1.00
Preferred stock, shares authorized	7.5	7.5
Preferred stock, shares issued	0.0	0.0
Common stock, par value	$ 0.15	$ 0.15
Common stock, shares authorized	1,000.0	1,000.0
Common stock, shares issued	297.2	297.2
Common stock, shares outstanding	230.1	234.7
Treasury stock, shares	67.1	62.5