Investments - Schedule of Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fixed Maturities Available for Sale, Amortized Cost
Due in one year or less	$ 147,457
Due after one year through five years	682,932
Due after five years through ten years	1,397,473
Due after ten years through twenty years	4,701,591
Due after twenty years	7,861,000
Mortgage-backed and asset-backed securities	204,648
Cost or Amortized Cost	14,995,101	$ 14,188,050
Fixed Maturities Available for Sale, Fair Value
Due in one year or less	149,495
Due after one year through five years	720,186
Due after five years through ten years	1,567,972
Due after ten years through twenty years	5,519,917
Due after twenty years	8,789,769
Mortgage-backed and asset-backed securities	221,986
Total fixed maturities available for sale, Fair Value	$ 16,969,325	$ 15,245,861