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Investments - Summary of Net Investment Income (Detail) - Successor [Member] - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net Realized and Unrealized Gain (Loss) on Trading Securities |  |  |  |  |  |  |  |  | $ 18,242 | $ 27,654 | $ 12,615
Trading Securities, Realized Gain (Loss) |  |  |  |  |  |  |  |  | 4,854 | 7,474 | 4,966
Fixed income securities |  |  |  |  |  |  |  |  | 337,454 | 288,554 | 257,404
Short-term investments |  |  |  |  |  |  |  |  | 7,898 | 1,505 | 299
Loans |  |  |  |  |  |  |  |  | 520 | 337 | 420
Investment expense |  |  |  |  |  |  |  |  | (8,098) | (9,347) | (8,786)
Securities available-for-sale and short-term |  |  |  |  |  |  |  |  | 337,774 | 281,049 | 249,337
Other investments |  |  |  |  |  |  |  |  | 23,179 | 32,318 | 16,952
Total net investment income | $ 107,057 | $ 87,177 | $ 85,160 | $ 81,559 | $ 90,871 | $ 90,917 | $ 70,758 | $ 60,821 | 360,953 | 313,367 | 266,289
Gains (losses) on securities held as of reporting date [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other investments |  |  |  |  |  |  |  |  | $ 13,388 | $ 20,180 | $ 7,649

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- Definition Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 12 -Sentence Column A + Details
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