Net Investment Income and Realized Investment Gains (Losses) - Summary of Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net Investment Income [Line Items]
Net Investment Income, Total	$ 292,346	$ 208,669
Debt Securities
Net Investment Income [Line Items]
Net Investment Income, Total	147,557	140,095
Dividends on Equity Securities [Member]
Net Investment Income [Line Items]
Net Investment Income, Total	8,292	9,440
Mortgage Loans
Net Investment Income [Line Items]
Net Investment Income, Total	63,199	63,868
Real Estate
Net Investment Income [Line Items]
Net Investment Income, Total	1,855	4,283
Other Invested Assets
Net Investment Income [Line Items]
Net Investment Income, Total	4,958	5,128
Options [Member]
Net Investment Income [Line Items]
Net Investment Income, Total	$ 66,485	$ (14,145)