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, 2018	Dec. 31, 2017	Dec. 31, 2016
Gain (Loss) on Securities [Line Items]
Realized investment gains (losses)	$ 18,174	$ 104,595	$ 46,607
Other-than-temporary impairment losses	(1,243)	(13,386)	(17,667)
Debt Securities
Gain (Loss) on Securities [Line Items]
Realized investment gains (losses)	10,903	27,061	16,705
Other-than-temporary impairment losses	(1,243)	(6,416)	(94)
Equity Securities
Gain (Loss) on Securities [Line Items]
Realized investment gains (losses)	0	56,528	33,348
Other-than-temporary impairment losses	0	(6,970)	(17,573)
Mortgage Loans
Gain (Loss) on Securities [Line Items]
Realized investment gains (losses)	(4,798)	(7,700)	405
Real Estate
Gain (Loss) on Securities [Line Items]
Realized investment gains (losses)	12,076	28,853	2,188
Other Invested Assets
Gain (Loss) on Securities [Line Items]
Realized investment gains (losses)	$ (7)	$ (147)	$ (6,039)