Investment in Securities - Change in Net Unrealized Gains (Losses) on Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Adjustments for
Deferred policy acquisition costs			$ 34,853
Change in net unrealized gains (losses) on debt securities, net of tax	$ 98,360	$ (36,388)	183,874	$ (127,721)
Unrealized gains on equity securities
Net gains on equity securities	67,060	57,162	273,437	23,992
Bonds
Schedule of Available-for-sale Securities [Line Items]
Bonds available-for-sale: change in unrealized gains (losses)			279,618	(210,413)
Adjustments for
Deferred policy acquisition costs			(34,853)	41,962
Participating policyholders’ interest			(12,811)	11,924
Deferred federal income tax benefit (expense)			(48,080)	28,806
Change in net unrealized gains (losses) on debt securities, net of tax			183,874	(127,721)
Equity Securities
Unrealized gains on equity securities
Unrealized gains on equity securities	51,933	46,087	254,955	11,862
Net gains on equity securities sold	15,127	11,075	18,482	12,130
Net gains on equity securities	$ 67,060	$ 57,162	$ 273,437	$ 23,992