Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Amortized Cost	$ 49,967	$ 76,875
After one year through five years, Amortized Cost	198,025	256,614
After five years through ten years, Amortized Cost	110,460	81,311
After ten years, Amortized Cost	229,782	189,926
Held-to-maturity, Amortized Cost, Totals	599,421	613,878
Within one year, Fair Value	50,348	76,861
After one year through five years, Fair Value	202,109	257,530
After five years through ten years, Fair Value	113,877	81,755
After ten years, Fair Value	243,279	191,213
Held-to-maturity, Fair Value, Totals	621,343	616,772
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Amortized Cost	10,967	8,102
Held-to-maturity, Amortized Cost, Totals	10,967	8,102
Without single maturity, Fair Value	11,511	8,349
Held-to-maturity, Fair Value, Totals	11,511	8,349
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Amortized Cost	220	1,050
Held-to-maturity, Amortized Cost, Totals	220	1,050
Without single maturity, Fair Value	219	1,064
Held-to-maturity, Fair Value, Totals	$ 219	$ 1,064