Net Investment Income and Realized Investment Gains (Losses) - Summary of Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net Investment Income [Line Items]
Net Investment Income, Total	$ 115,284	$ 292,346
Bonds
Net Investment Income [Line Items]
Net Investment Income, Total	146,119	147,557
Equity securities
Net Investment Income [Line Items]
Net Investment Income, Total	7,847	8,292
Mortgage loans
Net Investment Income [Line Items]
Net Investment Income, Total	59,328	63,199
Real estate
Net Investment Income [Line Items]
Net Investment Income, Total	1,713	1,855
Other invested assets
Net Investment Income [Line Items]
Net Investment Income, Total	8,372	4,958
Equity indexed options
Net Investment Income [Line Items]
Net Investment Income, Total	$ (108,095)	$ 66,485