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Marketable Securities - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items] |  |  |
Marketable securities | $ 330 | $ 431 | $ 335
U.S. Treasury Debt Securities [Member] |  |  |
Financing Receivable, Recorded Investment [Line Items] |  |  |
Marketable securities | $ 330 | $ 431 | $ 335
Weighted average maturity period | 1 year 3 months 18 days |  |
Aggregate amortized cost basis of securities | $ 332 |  |

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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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