Net Investment Income and Realized Investment Gains (Losses) - Summary of Realized Investment Gains (Losses) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	$ 31,933	$ (1,276)	$ 31,943	$ 4,775
Bonds
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	6,075	1,858	11,288	8,595
Mortgage loans
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	(2,097)	(2,279)	(2,186)	(2,833)
Real estate
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	27,388	(886)	24,502	(1,000)
Other invested assets
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	$ 567	$ 31	$ (1,661)	$ 13