Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Within one year, Amortized Cost	$ 67,547
After one year through five years, Amortized Cost	248,804
After five years through ten years, Amortized Cost	86,353
After ten years, Amortized Cost	190,527
Held-to-maturity, Amortized Cost, Totals	604,056	$ 613,878
Within one year, Fair Value	67,672
After one year through five years, Fair Value	251,430
After five years through ten years, Fair Value	88,108
After ten years, Fair Value	196,824
Held-to-maturity, Fair Value, Totals	615,217	616,772
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	9,824
Held-to-maturity, Amortized Cost, Totals	9,824	8,102
Held-to-maturity, Fair Value	10,164
Held-to-maturity, Fair Value, Totals	10,164	8,349
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	1,001
Held-to-maturity, Amortized Cost, Totals	1,001	1,050
Held-to-maturity, Fair Value	1,019
Held-to-maturity, Fair Value, Totals	$ 1,019	$ 1,064