Net Investment Income and Realized Investment Gains (Losses) - Summary of Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net Investment Income [Line Items]
Net Investment Income, Total	$ 257,730	$ 246,741	$ 550,076	$ 455,410
Bonds
Net Investment Income [Line Items]
Net Investment Income, Total	150,907	142,529	298,464	282,624
Dividends on equity securities
Net Investment Income [Line Items]
Net Investment Income, Total	9,015	10,898	17,307	20,338
Mortgage loans
Net Investment Income [Line Items]
Net Investment Income, Total	63,158	58,999	126,357	122,867
Real estate
Net Investment Income [Line Items]
Net Investment Income, Total	4,508	4,212	6,363	8,495
Other invested assets
Net Investment Income [Line Items]
Net Investment Income, Total	7,017	8,325	11,975	13,453
Options
Net Investment Income [Line Items]
Net Investment Income, Total	$ 23,125	$ 21,778	$ 89,610	$ 7,633