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Investments - Summary of Net Investment Income (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net Realized and Unrealized Gain (Loss) on Trading Securities |  |  |  |  |  |  |  |  | $ (3,035) | $ 18,242 | $ 27,654
Trading Securities, Realized Gain (Loss) |  |  |  |  |  |  |  |  | 615 | 4,854 | 7,474
Fixed income securities |  |  |  |  |  |  |  |  |  | 337,454 | 288,554
Investment expense |  |  |  |  |  |  |  |  | (6,599) | (8,098) | (9,347)
Securities available-for-sale and short-term |  |  |  |  |  |  |  |  | 270,525 | 337,774 | 281,049
Total net investment income | $ 37,483 | $ 58,332 | $ 66,662 | $ 110,240 | $ 107,057 | $ 87,177 | $ 85,160 | $ 81,559 | 272,717 | 360,953 | 313,367
Gains (losses) on securities held as of reporting date [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other Investments Income |  |  |  |  |  |  |  |  | (3,650) | 13,388 | 20,180
Available-for-sale Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Fixed income securities |  |  |  |  |  |  |  |  | 265,380 |  |
Short-term Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Fixed income securities |  |  |  |  |  |  |  |  | 11,014 | 7,898 | 1,505
Financing Receivable [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Fixed income securities |  |  |  |  |  |  |  |  | 730 | 520 | 337
Other Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Fixed income securities |  |  |  |  |  |  |  |  | $ 2,192 | $ 23,179 | $ 32,318

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