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Securities - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items] |  |  |
Realized gains and losses on sales | $ 10.1 | $ 13.6 | $ 85.7
Investments pledged, carrying value | $ 81.8 | $ 82.9 |
Minimum [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Underwater security | 10 years |  |
Loss position existed | 3 years |  |

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