Investments - Summary of Carrying Amounts and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 84,032	$ 49,967
After one year through five years, Carrying Amount	184,810	198,025
After five years through ten years, Carrying Amount	91,537	110,460
After ten years, Carrying Amount	237,386	229,782
Held-to-maturity, Carrying Amount, Totals	608,315	599,421
Within one year, Fair Value	84,737	50,348
After one year through five years, Fair Value	188,832	202,109
After five years through ten years, Fair Value	94,545	113,877
After ten years, Fair Value	252,304	243,279
Held-to-maturity, Fair Value, Totals	631,688	621,343
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	10,352	10,967
Without single maturity, Fair Value	11,072	11,511
Held-to-maturity, Fair Value, Totals	11,072	11,511
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	198	220
Without single maturity, Fair Value	198	219
Held-to-maturity, Fair Value, Totals	$ 198	$ 219