Scheduled Maturity Dates for Fixed Maturity Securities by Number of Years Until Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value
Due in One Year or Less	$ 228,479
Due after One Year Through Five Years	809,718
Due After Five Years Through Ten Years	359,745
Due After Ten Years	707,732
Mortgage-Backed and Asset-Backed Securities	951,380
Total	3,057,054	$ 2,635,882
Amortized Cost
Due in One Year or Less	228,255
Due after One Year Through Five Years	809,195
Due After Five Years Through Ten Years	352,524
Due After Ten Years	686,166
Mortgage-Backed and Asset-Backed Securities	951,268
Total	$ 3,027,408	$ 2,628,225