Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Millions, $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts	$ 24.9	$ 22.5
Accumulated depreciation	1,233.4	1,206.6
Intangible assets, accumulated amortization	$ 777.6	$ 680.7
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	397.2
Common stock, shares outstanding	234.7	239.7
Treasury stock, shares	62.5	157.5