Marketable Securities - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Held To Maturity Securities [Line Items]
Marketable securities matured during the period and held in money market funds.	$ 80,000,000
Other-than-temporary impairment losses related to marketable securities	0	$ 0	$ 0
Net Interest (Income) Expense
Schedule Of Held To Maturity Securities [Line Items]
Interest income	$ 2,000,000	$ 1,300,000	$ 500,000

Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(b))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

+ References

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The amount by which the fair value of an investment in debt securities categorized as Held-to-maturity is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=82896770&loc=SL6284422-111562

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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