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Marketable Securities - Additional Information (Details) - USD ($) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Unrealized loss position for a period longer than twelve months | $ 6,300,000 |  | $ 6,300,000 |  |
Other-than-temporary impairment charges | 0 | $ 0 | 0 | $ 0 |
Marketable securities, due within one year | 33,455,000 |  | 33,455,000 |  | $ 37,121,000
Proceeds from sale of marketable securities | 0 | $ 0 | 0 | $ 0 |
Variable rate demand notes |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Marketable securities, due within one year | $ 5,600,000 |  | $ 5,600,000 |  |
Variable rate demand notes | Minimum |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Contractual maturities (in years) |  |  | 18 years |  |
Settlement basis period (in days) |  |  | 1 day |  |
Effective maturity period (in days) |  |  | 1 day |  |
Variable rate demand notes | Maximum |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Contractual maturities (in years) |  |  | 26 years |  |
Settlement basis period (in days) |  |  | 7 days |  |
Effective maturity period (in days) |  |  | 7 days |  |

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