Scheduled Maturity Dates for Fixed Maturity Securities by Number of Years Until Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value
Due in one year or less	$ 232,015
Due after one year through five years	820,868
Due after five years through ten years	322,289
Due after ten years	622,250
Mortgage-Backed and Asset-Backed Securities	973,512
Total	2,970,934	$ 3,057,054
Amortized Cost
Due in one year or less	232,571
Due after one year through five years	829,156
Due after five years through ten years	321,789
Due after ten years	617,317
Mortgage-Backed and Asset-Backed Securities	986,659
Total	$ 2,987,492	$ 3,027,408