Investment in Securities - Change in Net Unrealized Gains (Losses) on Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Adjustments for
Deferred policy acquisition costs			$ 82,452
Change in net unrealized gains (losses) on debt securities, net of tax	$ (2,569)	$ (14,395)	181,305	$ (142,116)
Unrealized gains on equity securities
Net gains on equity securities	8,589	126,495	282,026	150,487
Bonds
Schedule of Available-for-sale Securities [Line Items]
Bonds available-for-sale: change in unrealized gains (losses)			326,945	(232,433)
Adjustments for
Deferred policy acquisition costs			(82,587)	38,871
Participating policyholders’ interest			(14,989)	13,975
Deferred federal income tax benefit (expense)			(48,064)	37,471
Change in net unrealized gains (losses) on debt securities, net of tax			181,305	(142,116)
Equity Securities
Unrealized gains on equity securities
Unrealized gains on equity securities	3,605	133,825	258,209	145,687
Net gains (losses) on equity securities sold	4,984	(7,330)	23,817	4,800
Net gains on equity securities	$ 8,589	$ 126,495	$ 282,026	$ 150,487