Investment in Securities - Change in Net Unrealized Gains (Losses) on Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Adjustments for
Deferred policy acquisition costs	$ 87,003	$ (51,920)	$ 2,083
Change in net unrealized gains (losses) on debt securities, net of tax	184,156	(136,261)	169,740
Net gains (losses) on equity securities
Net gains (losses) on equity securities	422,535	(107,188)	0
Debt Securities
Debt Securities, Available-for-sale [Line Items]
Bonds available-for-sale: change in unrealized gains (losses)	335,473	(233,465)	53,115
Adjustments for
Deferred policy acquisition costs	(87,003)	51,920	(2,083)
Participating policyholders’ interest	(16,056)	11,157	(7,086)
Deferred federal income tax benefit (expense)	(48,258)	34,127	(15,516)
Change in net unrealized gains (losses) on debt securities, net of tax	184,156	(136,261)	$ 28,430
Equity Securities
Net gains (losses) on equity securities
Unrealized gains (losses) on equity securities	375,395	(109,230)
Net gains on equity securities sold	47,140	2,042
Net gains (losses) on equity securities	$ 422,535	$ (107,188)