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Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Realized investment gains (losses), description	Net realized gains and losses on sales of investments were not material in 2019, 2018 and 2017.	Net realized gains and losses on sales of investments were not material in 2019, 2018 and 2017.	Net realized gains and losses on sales of investments were not material in 2019, 2018 and 2017.

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Realized Investment Gains Losses Description.

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