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Investments - Summary of Carrying Amounts and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Schedule Of Held To Maturity Securities [Line Items] |  |
Within one year, Carrying Amount | $ 54,316 | $ 49,967
After one year through five years, Carrying Amount | 195,706 | 198,025
After five years through ten years, Carrying Amount | 107,347 | 110,460
After ten years, Carrying Amount | 220,345 | 229,782
Held-to-maturity, Carrying Amount, Totals | 585,130 | 599,421
Within one year, Fair Value | 54,794 | 50,348
After one year through five years, Fair Value | 204,289 | 202,109
After five years through ten years, Fair Value | 113,643 | 113,877
After ten years, Fair Value | 240,860 | 243,279
Held-to-maturity, Fair Value, Totals | 621,654 | 621,343
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Without single maturity, Carrying Amount | 7,261 | 10,967
Held-to-maturity, Carrying Amount, Totals | 7,261 |
Without single maturity, Fair Value | 7,906 | 11,511
Held-to-maturity, Fair Value, Totals | 7,906 | 11,511
Asset-Backed Securities [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Without single maturity, Carrying Amount | 155 | 220
Held-to-maturity, Carrying Amount, Totals | 155 |
Without single maturity, Fair Value | 162 | 219
Held-to-maturity, Fair Value, Totals | $ 162 | $ 219

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- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing in next rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. + References No definition available. + Details
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- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing after tenth rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. + References No definition available. + Details
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- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing in sixth through tenth rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. + References No definition available. + Details
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