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Investments - Additional Information (Detail) - USD ($) | Sep. 30, 2016 | Dec. 31, 2015
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Investments, Debt and Equity Securities [Abstract] |  |
Net unrealized gains on investments included in "AOCI" | $ 1,500,000,000 | $ 996,000,000
Additional other than temporary impairment losses | 0 |
Amortized cost of marketable securities unrealized loss position | $ 0 |

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