Scheduled Maturity Dates for Fixed Maturity Securities by Number of Years Until Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Total	$ 2,996,737
Amortized Cost
Amortized Cost	3,035,103
Fixed Maturities
Fair Value
Due in one year or less	179,367
Due after one year through five years	715,620
Due after five years through ten years	345,661
Due after ten years	553,801
Mortgage-Backed and Asset-Backed Securities	1,195,024
Total	2,989,473	$ 3,057,054
Amortized Cost
Due in one year or less	180,246
Due after one year through five years	725,231
Due after five years through ten years	348,557
Due after ten years	556,551
Mortgage-Backed and Asset-Backed Securities	1,217,250
Amortized Cost	$ 3,027,835	$ 3,027,408