Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net earnings	$ 126,883	$ 890,201	$ 54,654
Adjustments to reconcile net earnings to net cash provided by (used in) operating activities:
Depreciation and amortization	160,002	119,513	127,374
Change in the fair value of equity securities	110,459	(709,695)	228,994
Net realized capital (gains) losses	(3,098)	6,551	3,241
Change in allowance for credit losses on available for sale securities	8,029
Change in allowance for credit losses on available for sale securities		19,660	1,328
Net adjustments	909,951	(175,796)	347,769
Net cash provided by (used in) operating activities	1,036,834	714,405	402,423
Cash flows from investing activities
Purchases of equity securities	(1,974,625)	(520,961)	(1,041,887)
Purchases of debt securities	(6,721,073)	(6,726,809)	(4,716,070)
Sales of debt securities	4,042,915	3,645,307	3,647,156
Maturities and redemptions of debt securities	1,664,285	1,166,841	1,526,060
Sales of equity securities	1,659,234	2,296,371	1,609,647
Net (purchase) sale of short-term investments	208,263	(21,564)	(316,366)
Purchases of property and equipment	(37,519)	(47,572)	(23,286)
Other, net	(12,973)	(40,218)	(86,478)
Net cash provided by (used in) investing activities	(1,284,447)	(476,539)	345,432
Cash flows from financing activities
Repayment of senior notes	(307,095)	0	0
Treasury stock acquisitions	(194,762)	(144,422)	(491,633)
Proceeds from issuance of senior notes	499,335	0	0
Debt issue costs paid	(4,550)	0	0
Cash dividends paid	(215,013)	0	(153,967)
Other, net	(27,912)	(40,530)	4,512
Net cash provided by (used in) financing activities	(164,008)	(103,214)	(536,684)
Net (decrease) increase in cash	(387,656)	140,335	200,388
Cash at beginning of period	1,179,098	1,038,763	838,375
Cash at end of period	791,442	1,179,098	1,038,763
Cash paid during the period for:
Interest paid	91,832	97,016	88,345
Income taxes paid (refunds received)	90,828	61,786	33,677
Parent Company
Cash flows from operating activities
Net earnings	126,883	890,201	54,654
Adjustments to reconcile net earnings to net cash provided by (used in) operating activities:
Equity in undistributed net (earnings) losses of consolidated subsidiaries	(226,590)	(989,486)	(130,050)
Depreciation and amortization	4,064	2,249	1,750
Change in the fair value of equity securities	54,647	(3,937)	44,780
Net realized capital (gains) losses	(6,522)	13,497	265
Change in allowance for credit losses on available for sale securities	57
Change in allowance for credit losses on available for sale securities		0	0
Increase (decrease) in other liabilities and taxes payable	(56,298)	24,593	(84,803)
Net adjustments	(230,642)	(953,084)	(168,058)
Net cash provided by (used in) operating activities	(103,759)	(62,883)	(113,404)
Cash flows from investing activities
Purchases of equity securities	(763,986)	0	(31,125)
Purchases of debt securities	(497,058)	(194,706)	(3,418)
Sales of debt securities	606,110	1,186	29,189
Maturities and redemptions of debt securities	32,383	13,344	53
Sales of equity securities	142,065	64,016	360,579
Net (purchase) sale of short-term investments	49,154	162,161	(195,983)
Purchases of property and equipment	(9)	(124)	0
Other, net	(8,731)	(33,245)	(4,926)
Net cash provided by (used in) investing activities	(440,072)	12,632	154,369
Cash flows from financing activities
Repayment of senior notes	(307,095)	0	0
Treasury stock acquisitions	(194,762)	(144,422)	(491,633)
Proceeds from issuance of senior notes	499,335	0	0
Debt issue costs paid	(4,550)	0	0
Cash dividends paid	(215,013)	0	(153,967)
Capital contributions to consolidated subsidiaries	(87,323)	(15,267)	(175,653)
Distributions from consolidated subsidiaries	865,000	215,000	751,471
Other, net	4,798	(4,707)	7,868
Net cash provided by (used in) financing activities	560,390	50,604	(61,914)
Net (decrease) increase in cash	16,559	353	(20,949)
Cash at beginning of period	684	331	21,280
Cash at end of period	17,243	684	331
Cash paid during the period for:
Interest paid	49,036	51,375	51,375
Income taxes paid (refunds received)	$ 17,255	$ (4,189)	$ (8,338)