Net Investment Income and Realized Investment Gains (Losses) - Summary of Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net Investment Income [Line Items]
Net Investment Income, Total	$ 246,620	$ 285,532	$ 796,696	$ 740,942
Bonds
Net Investment Income [Line Items]
Net Investment Income, Total	154,322	141,045	452,786	423,669
Dividends on equity securities
Net Investment Income [Line Items]
Net Investment Income, Total	7,985	10,387	25,292	30,725
Mortgage loans
Net Investment Income [Line Items]
Net Investment Income, Total	64,753	64,741	191,110	187,608
Real estate
Net Investment Income [Line Items]
Net Investment Income, Total	1,005	2,783	7,310	11,278
Other invested assets
Net Investment Income [Line Items]
Net Investment Income, Total	12,277	15,633	24,310	29,086
Options
Net Investment Income [Line Items]
Net Investment Income, Total	$ 6,278	$ 50,943	$ 95,888	$ 58,576