Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Thousands, $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Accounts receivable, allowances for doubtful accounts and billing adjustments	$ 4.0	$ 4.8
Computer software, accumulated amortization	831.7	757.4
Property and equipment, accumulated depreciation and amortization	$ 521.7	$ 480.7
Common stock, par value	$ 0.10	$ 0.10
Common stock, authorized	600,000	600,000
Common stock, issued	202,765	202,765
Common stock, outstanding	184,088	183,451
Treasury stock, shares	18,677	19,314
Other intangible assets
Intangible assets, accumulated amortization	$ 557.0	$ 420.6
Contract Acquisition Costs
Intangible assets, accumulated amortization	$ 337.9	$ 309.7