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Marketable Securities - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Net realized losses on investments	$ (1)
Realized investment gains (losses), description		Realized gains and losses on sales of investments were not material in 2017 and 2016.	Realized gains and losses on sales of investments were not material in 2017 and 2016.

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The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.3(a))
-URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263

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Realized Investment Gains Losses Description

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