Income Taxes - Summary of Deferred Tax Valuation Allowance (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Balance, beginning of period	$ 406.0	$ 382.4	$ 385.6
Additions (reductions) recorded in the provision for income taxes	(9.0)	23.6	(5.8)
Changes in tax rates	(140.9)
Business acquired			2.6
Balance, end of period	$ 256.1	$ 406.0	$ 382.4

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant