Investments - Summary of Carrying Amounts and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 59,898	$ 49,967
After one year through five years, Carrying Amount	195,398	198,025
After five years through ten years, Carrying Amount	80,484	110,460
After ten years, Carrying Amount	230,999	229,782
Held-to-maturity, Carrying Amount, Totals	576,191	599,421
Within one year, Fair Value	60,504	50,348
After one year through five years, Fair Value	204,082	202,109
After five years through ten years, Fair Value	84,789	113,877
After ten years, Fair Value	249,767	243,279
Held-to-maturity, Fair Value, Totals	609,281	621,343
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	9,227	10,967
Held-to-maturity, Carrying Amount, Totals	9,227
Without single maturity, Fair Value	9,947	11,511
Held-to-maturity, Fair Value, Totals	9,947	11,511
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	185	220
Held-to-maturity, Carrying Amount, Totals	185
Without single maturity, Fair Value	192	219
Held-to-maturity, Fair Value, Totals	$ 192	$ 219