Net Investment Income and Realized Investment Gains (Losses) - Summary of Net Investment Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Investment Income [Line Items]
Net investment income	$ 1,077,406	$ 858,367	$ 966,077
Bonds
Net Investment Income [Line Items]
Net investment income	602,054	566,513	541,772
Equity Securities
Net Investment Income [Line Items]
Net investment income	33,502	39,193	38,730
Mortgage Loans
Net Investment Income [Line Items]
Net investment income	254,720	258,102	245,116
Real Estate
Net Investment Income [Line Items]
Net investment income	8,849	13,533	12,672
Other Invested Assets
Net Investment Income [Line Items]
Net investment income	33,301	35,977	36,732
Equity Indexed Options
Net Investment Income [Line Items]
Net investment income	$ 144,980	$ (54,951)	$ 91,055