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Investments (Additional Information) (Details) - USD ($) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Schedule of Investments [Line Items] |  |  |  |
Net unrealized gains (losses) on available-for-sale securities, net of tax | $ 3,250,000 | $ (17,826,000) | $ (10,300,000) | $ (6,085,000)
Deferred Tax Liabilities, Investments | $ 890,000,000 |  |  |
Deferred Tax Assets, Unrealized Losses on Available-for-Sale Securities, Gross |  | $ 4,900,000 |  |
Unrealized Loss as a Percent of Amortized Cost | 1.00% | 3.00% |  |
Security Owned And Pledged As Collateral Amortized Cost | $ 709,000,000 | $ 734,100,000 |  |
Security Owned and Pledged as Collateral, Fair Value | 712,400,000 | 714,400,000 |  |
Marketable Securities, Unrealized Gain (Loss) | $ 928,000 | $ (50,000) |  |

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