Net Investment Income and Realized Investment Gains (Losses) - Summary of Realized Investment Gains (Losses) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	$ 4,148	$ 2,947
Bonds
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	5,478	2,602
Mortgage loans
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	0	455
Real estate
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	(1,307)	(158)
Other invested assets
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	$ (23)	$ 48