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Securities - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items] |  |  |
Realized gains and losses on sales | $ 47.0 | $ 57.9 | $ 91.0
Investments with a carrying value and fair value | $ 189.3 | $ 176.9 |
Minimum [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Underwater security | 10 years |  |
Loss position existed | 3 years |  |

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