Net Investment Income and Realized Investment Gains (Losses) - Summary of Realized Investment Gains (Losses) and Other Than Temporary Impairment Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	$ 37,719	$ 18,174
Net realized investment gains (losses)			$ 104,595
Bonds
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	16,361	10,903
Net realized investment gains (losses)			27,061
Equity Securities
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	0	0
Net realized investment gains (losses)			56,528
Mortgage Loans
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	(2,412)	(4,798)
Net realized investment gains (losses)			(7,700)
Real Estate
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	25,555	12,076
Net realized investment gains (losses)			28,853
Other Invested Assets
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	$ (1,785)	$ (7)
Net realized investment gains (losses)			$ (147)