Scheduled Maturity Dates for Fixed Maturity Securities by Number of Years Until Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value
Due in one year or less	$ 218,780
Due after one year through five years	708,302
Due after five years through ten years	333,260
Due after ten years	604,407
Mortgage-Backed and Asset-Backed Securities	933,436
Total	2,798,185	$ 2,635,882
Amortized Cost
Due in one year or less	216,537
Due after one year through five years	697,520
Due after five years through ten years	324,180
Due after ten years	583,550
Mortgage-Backed and Asset-Backed Securities	930,315
Total	$ 2,752,102	$ 2,628,225