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Summary of Significant Accounting Policies - Income Taxes (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Income Taxes [Line Items]
Total amount of unrecognized tax benefits relating to uncertain tax positions	$ 31.8
Amount of unrecognized tax benefits change from year end relating to uncertain tax positions	(3.4)
Decrease in unrecognized tax benefits resulting from expiration of statute of limitations periods	6.2
Maximum
Income Taxes [Line Items]
Reasonably possible reduction to the balance of unrecognized tax benefits in succeeding 12 months	$ 13.7

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Income taxes.

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Amount of decrease reasonably possible in the next twelve months for the unrecognized tax benefit.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (d)(3)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32718-109319

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Amount of unrecognized tax benefits.

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-Topic 740
-SubTopic 10
-Section 50
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-Name Accounting Standards Codification
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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

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-Name Accounting Standards Codification
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