Net Investment Income and Realized Investment Gains (Losses) - Summary of Net Investment Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Investment Income [Line Items]
Net Investment Income, Total	$ 858,367	$ 966,077	$ 860,235
Debt Securities
Net Investment Income [Line Items]
Net Investment Income, Total	566,513	541,772	551,849
Equity Securities
Net Investment Income [Line Items]
Net Investment Income, Total	39,193	38,730	38,680
Mortgage Loans
Net Investment Income [Line Items]
Net Investment Income, Total	258,102	245,116	211,972
Real Estate
Net Investment Income [Line Items]
Net Investment Income, Total	13,533	12,672	6,743
Other Invested Assets
Net Investment Income [Line Items]
Net Investment Income, Total	35,977	36,732	22,122
Options
Net Investment Income [Line Items]
Net Investment Income, Total	$ (54,951)	$ 91,055	$ 28,869