Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Trade accounts receivable, allowance for doubtful accounts	$ 11.9	$ 11.2
Preferred stock:
Par value (in dollars per share)	$ 0.01	$ 0.01
Authorized (in shares)	5,000,000	5,000,000
Issued (in shares)	0	0
Outstanding (in shares)	0	0
Common stock:
Par value (in dollars per share)	$ 0.01	$ 0.01
Authorized (in shares)	400,000,000	400,000,000
Issued (in shares)	287,591,536	289,161,924
Outstanding (in shares)	287,591,536	289,161,924

A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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