Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES
Net income (loss)	$ 374,607	$ 320,087	$ (5,728)
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	68,463	64,730	58,791
Net realized investment (gains) losses	(85,731)	(222,793)	133,520
Decrease (increase) in premiums receivable	13,946	(49,832)	(80,979)
Decrease (increase) in reinsurance recoverables	30,127	142,314	(164,739)
Changes in current and deferred income taxes	31,236	72,545	(13,898)
Increase in deferred policy acquisition costs	(13,828)	(18,035)	(16,980)
Increase in loss and loss adjustment expense reserves	70,049	91,843	318,799
Increase in unearned premiums	50,326	119,366	134,254
Increase in accounts payable and accrued expenses	51,988	26,493	6,586
Share-based compensation	142	123	145
Other, net	14,291	(27,163)	13,663
Net cash provided by operating activities	605,616	519,678	383,434
Proceeds from Sale and Maturity of Debt Securities, Available-for-sale [Abstract]
Purchases	(878,213)	(491,795)	(706,224)
Proceeds from Sale of Debt Securities, Available-for-sale	168,087	136,560	189,306
Calls or maturities	250,878	316,860	334,626
Equity securities available for sale in nature
Purchases	(1,230,785)	(1,174,759)	(1,026,827)
Sales	1,174,540	1,088,701	954,755
Calls	3,880	0	0
Changes in securities payable and receivable	873	(2,536)	4,035
Decrease (increase) in short-term investments	118,830	(240,391)	45,747
Purchase of fixed assets	(39,953)	(40,088)	(27,959)
Other, net	20,577	6,247	10,105
Net cash used in investing activities	(411,286)	(401,201)	(222,436)
CASH FLOWS FROM FINANCING ACTIVITIES
Dividends paid to shareholders	(139,640)	(139,071)	(138,478)
Proceeds from stock options exercised	0	701	358
Payments on finance lease obligations	(609)	0
Net cash used in financing activities	(140,249)	(138,370)	(138,120)
Net increase (decrease) in cash	54,081	(19,893)	22,878
Cash:
Beginning of year	294,398	314,291	291,413
End of year	348,479	294,398	314,291
SUPPLEMENTAL CASH FLOW DISCLOSURE
Interest paid	16,599	16,586	16,586
Income taxes paid (refunded), net	$ 52,659	$ (14,564)	$ (10,989)