RLI (RLI) 10-Q 25 Oct 23 2023 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2023	Oct. 16, 2023
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2023
Entity File Number	001-09463
Entity Registrant Name	RLI Corp
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	37-0889946
Entity Address, Address Line One	9025 North Lindbergh Drive,
Entity Address, City or Town	Peoria,
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	61615
City Area Code	309
Local Phone Number	692-1000
Title of 12(b) Security	Common Stock $0.01 par value
Trading Symbol	RLI
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		45,625,771
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3
Entity Central Index Key	0000084246
Amendment Flag	false

Condensed Consolidated Statemen

Condensed Consolidated Statements of Earnings and Comprehensive Earnings - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Statement [Abstract]
Net premiums earned	$ 318,409	$ 291,468	$ 948,412	$ 843,430
Net investment income	31,963	21,270	87,835	57,625
Net realized gains	6,558	573,170	26,758	591,562
Net unrealized gains (losses) on equity securities	(25,236)	(26,414)	15,474	(155,218)
Consolidated revenue	331,694	859,494	1,078,479	1,337,399
Losses and settlement expenses	189,558	165,089	457,989	388,527
Policy acquisition costs	103,013	96,977	307,083	271,879
Insurance operating expenses	21,591	20,606	70,002	58,794
Interest expense on debt	1,873	2,013	5,928	6,034
General corporate expenses	2,372	2,755	10,805	8,553
Total expenses	318,407	287,440	851,807	733,787
Equity in earnings (loss) of unconsolidated investees	1,732	(17,352)	7,169	3,061
Earnings before income taxes	15,019	554,702	233,841	606,673
Income tax expense	1,483	114,809	43,842	121,096
Net earnings	13,536	439,893	189,999	485,577
Other comprehensive earnings (loss), net of tax	(56,834)	(81,248)	(38,848)	(294,392)
Comprehensive earnings (loss)	$ (43,298)	$ 358,645	$ 151,151	$ 191,185
Basic net earnings per share	$ 0.30	$ 9.69	$ 4.17	$ 10.71
Diluted net earnings per share	$ 0.29	$ 9.61	$ 4.12	$ 10.61
Weighted average number of common shares outstanding:
Basic	45,622	45,379	45,581	45,347
Diluted	46,065	45,775	46,067	45,775

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
ASSETS
Available-for-sale, at fair value (amortized cost of $3,046,785 and allowance for credit losses of $704 at 9/30/23) (amortized cost of $2,945,273 and allowance for credit losses of $339 at 12/31/22)	$ 2,717,627	$ 2,666,950
Equity securities, at fair value (cost - $347,772 at 9/30/23 and $328,019 at 12/31/22)	534,436	498,382
Short-term investments, at cost which approximates fair value	125,032	36,229
Other invested assets	59,730	47,922
Cash	18,445	22,818
Total investments and cash	3,455,270	3,272,301
Accrued investment income	25,328	21,259
Premiums and reinsurance balances receivable, net of allowances for uncollectible amounts of $19,415 at 9/30/23 and $18,696 at 12/31/22	225,821	189,501
Ceded unearned premium	111,072	138,457
Reinsurance balances recoverable on unpaid losses and settlement expenses, net of allowances for uncollectible amounts of $11,131 at 9/30/23 and $11,250 at 12/31/22	820,954	740,089
Deferred policy acquisition costs	150,666	127,859
Property and equipment, at cost, net of accumulated depreciation of $74,416 at 9/30/23 and $68,633 at 12/31/22	48,596	49,573
Investment in unconsolidated investees	56,110	58,275
Goodwill and intangibles	53,562	53,562
Income taxes-deferred	51,403	40,269
Other assets	84,744	75,923
TOTAL ASSETS	5,083,526	4,767,068
Liabilities
Unpaid losses and settlement expenses	2,488,695	2,315,637
Unearned premiums	906,139	785,085
Reinsurance balances payable	56,570	61,100
Funds held	103,424	101,144
Income taxes-current	5,546
Debt	100,000	199,863
Accrued expenses	85,640	94,869
Other liabilities	38,421	32,029
TOTAL LIABILITIES	3,784,435	3,589,727
Shareholders' Equity
Common stock ($0.01 par value) (Shares authorized - 200,000,000) (68,555,985 shares issued, 45,625,771 shares outstanding at 9/30/23) (68,399,966 shares issued, 45,469,752 shares outstanding at 12/31/22)	686	684
Paid-in capital	360,453	352,391
Accumulated other comprehensive earnings (loss)	(267,924)	(229,076)
Retained earnings	1,598,875	1,446,341
Deferred compensation	12,617	12,015
Less: Treasury shares, at cost (22,930,214 shares at 9/30/23 and 12/31/22)	(405,616)	(405,014)
TOTAL SHAREHOLDERS' EQUITY	1,299,091	1,177,341
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 5,083,526	$ 4,767,068

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Statement Of Financial Position [Abstract]
Available-for-sale, amortized cost	$ 3,046,785	$ 2,945,273
Available-for-sale, allowance for credit losses	704	339
Equity securities, cost	347,772	328,019
Premiums and reinsurance balances receivable, allowances for uncollectible amounts	19,415	18,696
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	11,131	11,250
Property and equipment, accumulated depreciation	$ 74,416	$ 68,633
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	68,555,985	68,399,966
Common stock, shares outstanding (in shares)	45,625,771	45,469,752
Treasury stock, shares (in shares)	22,930,214	22,930,214

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Common stock	Paid-in Capital	Accumulated Other Comprehensive Earnings (Loss)	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings	Deferred Compensation	Treasury Stock at Cost	Cumulative Effect, Period of Adoption, Adjustment	Total
Stockholders' Equity Attributable to Parent, Beginning Balance at Dec. 31, 2021	$ 682	$ 343,742	$ 49,826		$ 1,228,110	$ 9,642	$ (402,641)		$ 1,229,361
Shares, Outstanding, Beginning Balance at Dec. 31, 2021	45,289,337
Increase Decrease In Stockholders Equity Roll Forward
Net Income (Loss)					47,923				47,923
Other comprehensive earnings (loss), net of tax			(115,581)						(115,581)
Deferred compensation						(973)	973
Share-based compensation		3,049							3,049
Share-based compensation (in shares)	26,605
Dividends and dividend equivalents					(11,330)				(11,330)
Stockholders' Equity Attributable to Parent, Ending Balance at Mar. 31, 2022	$ 682	346,791	(65,755)		1,264,703	8,669	(401,668)		1,153,422
Shares, Outstanding, Ending Balance at Mar. 31, 2022	45,315,942
Stockholders' Equity Attributable to Parent, Beginning Balance at Dec. 31, 2021	$ 682	343,742	49,826		1,228,110	9,642	(402,641)		1,229,361
Shares, Outstanding, Beginning Balance at Dec. 31, 2021	45,289,337
Increase Decrease In Stockholders Equity Roll Forward
Net Income (Loss)									485,577
Other comprehensive earnings (loss), net of tax									(294,392)
Stockholders' Equity Attributable to Parent, Ending Balance at Sep. 30, 2022	$ 683	352,129	(244,566)		1,678,746	9,047	(402,046)		1,393,993
Shares, Outstanding, Ending Balance at Sep. 30, 2022	45,378,171
Stockholders' Equity Attributable to Parent, Beginning Balance at Mar. 31, 2022	$ 682	346,791	(65,755)		1,264,703	8,669	(401,668)		1,153,422
Shares, Outstanding, Beginning Balance at Mar. 31, 2022	45,315,942
Increase Decrease In Stockholders Equity Roll Forward
Net Income (Loss)					(2,239)				(2,239)
Other comprehensive earnings (loss), net of tax			(97,563)						(97,563)
Deferred compensation						276	(276)
Share-based compensation	$ 1	2,370							2,371
Share-based compensation (in shares)	52,862
Dividends and dividend equivalents					(11,803)				(11,803)
Stockholders' Equity Attributable to Parent, Ending Balance at Jun. 30, 2022	$ 683	349,161	(163,318)		1,250,661	8,945	(401,944)		1,044,188
Shares, Outstanding, Ending Balance at Jun. 30, 2022	45,368,804
Increase Decrease In Stockholders Equity Roll Forward
Net Income (Loss)					439,893				439,893
Other comprehensive earnings (loss), net of tax			(81,248)						(81,248)
Deferred compensation						102	(102)
Share-based compensation		2,968							2,968
Share-based compensation (in shares)	9,367
Dividends and dividend equivalents					(11,808)				(11,808)
Stockholders' Equity Attributable to Parent, Ending Balance at Sep. 30, 2022	$ 683	352,129	(244,566)		1,678,746	9,047	(402,046)		1,393,993
Shares, Outstanding, Ending Balance at Sep. 30, 2022	45,378,171
Stockholders' Equity Attributable to Parent, Beginning Balance at Dec. 31, 2022	$ 684	352,391	(229,076)	$ (951)	1,446,341	12,015	(405,014)	$ (951)	1,177,341
Shares, Outstanding, Beginning Balance at Dec. 31, 2022	45,469,752
Increase Decrease In Stockholders Equity Roll Forward
Net Income (Loss)					98,811				98,811
Other comprehensive earnings (loss), net of tax			37,707						37,707
Deferred compensation						249	(249)
Share-based compensation	$ 1	2,863							2,864
Share-based compensation (in shares)	84,944
Dividends and dividend equivalents					(11,851)				(11,851)
Stockholders' Equity Attributable to Parent, Ending Balance at Mar. 31, 2023	$ 685	355,254	(191,369)		1,532,350	12,264	(405,263)		1,303,921
Shares, Outstanding, Ending Balance at Mar. 31, 2023	45,554,696
Stockholders' Equity Attributable to Parent, Beginning Balance at Dec. 31, 2022	$ 684	352,391	(229,076)	$ (951)	1,446,341	12,015	(405,014)	$ (951)	1,177,341
Shares, Outstanding, Beginning Balance at Dec. 31, 2022	45,469,752
Increase Decrease In Stockholders Equity Roll Forward
Net Income (Loss)									189,999
Other comprehensive earnings (loss), net of tax									(38,848)
Stockholders' Equity Attributable to Parent, Ending Balance at Sep. 30, 2023	$ 686	360,453	(267,924)		1,598,875	12,617	(405,616)		1,299,091
Shares, Outstanding, Ending Balance at Sep. 30, 2023	45,625,771
Stockholders' Equity Attributable to Parent, Beginning Balance at Mar. 31, 2023	$ 685	355,254	(191,369)		1,532,350	12,264	(405,263)		1,303,921
Shares, Outstanding, Beginning Balance at Mar. 31, 2023	45,554,696
Increase Decrease In Stockholders Equity Roll Forward
Net Income (Loss)					77,652				77,652
Other comprehensive earnings (loss), net of tax			(19,721)						(19,721)
Deferred compensation						243	(243)
Share-based compensation		2,402							2,402
Share-based compensation (in shares)	41,500
Dividends and dividend equivalents					(12,342)				(12,342)
Stockholders' Equity Attributable to Parent, Ending Balance at Jun. 30, 2023	$ 685	357,656	(211,090)		1,597,660	12,507	(405,506)		1,351,912
Shares, Outstanding, Ending Balance at Jun. 30, 2023	45,596,196
Increase Decrease In Stockholders Equity Roll Forward
Net Income (Loss)					13,536				13,536
Other comprehensive earnings (loss), net of tax			(56,834)						(56,834)
Deferred compensation						110	(110)
Share-based compensation	$ 1	2,797							2,798
Share-based compensation (in shares)	29,575
Dividends and dividend equivalents					(12,321)				(12,321)
Stockholders' Equity Attributable to Parent, Ending Balance at Sep. 30, 2023	$ 686	$ 360,453	$ (267,924)		$ 1,598,875	$ 12,617	$ (405,616)		$ 1,299,091
Shares, Outstanding, Ending Balance at Sep. 30, 2023	45,625,771

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Shareholders' Equity (Parenthetical) - $ / shares	3 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022
Statement Of Stockholders Equity [Abstract]
Cash dividends paid per common share	$ 0.27	$ 0.27	$ 0.26	$ 0.26	$ 0.26	$ 0.25

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Statement Of Cash Flows [Abstract]
Net cash provided by operating activities	$ 342,192	$ 282,886
Purchase of:
Fixed income securities, available-for-sale	(560,192)	(531,894)
Equity securities	(40,571)	(42,490)
Property and equipment	(5,451)	(4,432)
Other	(8,535)	(7,940)
Proceeds from sale of:
Fixed income securities - available-for-sale	26,158	46,448
Equity securities	36,708	59,264
Equity method investments	14,134	686,666
Other	550	2,298
Proceeds from call or maturity of:
Fixed income securities - available-for-sale	414,529	176,616
Net proceeds from sale (purchase) of short-term investments	(88,803)
Net cash provided by (used in) investing activities	(211,473)	384,536
Cash Flows from Financing Activities
Proceeds from issuance of debt	50,000
Payment of debt	(150,000)
Cash dividends paid	(36,490)	(34,913)
Proceeds from stock option exercises	1,398	2,198
Net cash used in financing activities	(135,092)	(32,715)
Net increase (decrease) in cash	(4,373)	634,707
Cash at the beginning of the period	22,818	88,804
Cash at September 30	$ 18,445	$ 723,511

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2023
Accounting Policies [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	NOTES TO UNAUDITED CONDENSED CONSOLIDATED INTERIM FINANCIAL STATEMENTS ​ 1. BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES ​ A. BASIS OF PRESENTATION ​ The unaudited condensed consolidated interim financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (GAAP) for interim financial reporting and with the instructions to Form 10-Q and Regulation S-X. Accordingly, they do not include all the disclosures required by GAAP for complete financial statements. As such, these unaudited condensed consolidated interim financial statements should be read in conjunction with our 2022 Annual Report on Form 10-K. Management believes that the disclosures are adequate to make the information presented not misleading, and all normal and recurring adjustments necessary to present fairly the financial position at September 30, 2023 and the results of operations of RLI Corp. (the Company) and subsidiaries for all periods presented have been made. The results of operations for any interim period are not necessarily indicative of the operating results for a full year. Certain reclassifications were made to 2022 to conform to the classifications used in the current year. ​ The preparation of the unaudited condensed consolidated interim financial statements requires management to make estimates and assumptions relating to the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the unaudited condensed consolidated interim financial statements and the reported amounts of revenue and expenses during the period. These estimates are inherently subject to change and actual results could differ significantly from these estimates. ​ B. ADOPTED ACCOUNTING STANDARDS ​ 2023-02—Investments – Equity Method and Joint Ventures (Topic 323): Accounting for Investments in Tax Credit Structures Using the Proportional Method ​ The amendments in this ASU permit the use of the proportional amortization method for investments in tax credits if certain conditions are met. Under the proportional amortization method, the initial cost of an investment is amortized in proportion to the amount of tax credits and other tax benefits received, with the amortization and tax credits presented as a component of income tax expense. Under previous guidance, equity investments in tax credit structures, other than qualified affordable housing projects, were accounted for using the equity method of accounting, which required the presentation of income, gains and losses, and tax credits in their respective line items of the statement of earnings. This ASU allows entities to make an accounting policy election on an individual tax credit program basis for all equity investments whose primary purpose is receiving income tax credits or other income tax benefits. When the proportional amortization method is selected, this amendment also requires a liability be recognized for delayed equity contributions that are unconditional or for contingent contributions when the contingent event becomes probable. ​ We adopted ASU 2023-02 ​ C. PROSPECTIVE ACCOUNTING STANDARDS ​ There are no prospective accounting standards which would have a material impact on our financial statements as of September 30, 2023. ​ D. REINSURANCE ​ Ceded unearned premiums and reinsurance balances recoverable on unpaid losses and settlement expenses are reported separately as an asset, rather than being netted with the related liability, since reinsurance does not relieve the Company of our liability to policyholders. Such balances are subject to the credit risk associated with the individual reinsurer. We continually monitor the financial condition of our reinsurers and actively follow up on any past due or disputed amounts. As part of our monitoring efforts, we review reinsurers’ annual financial statements and Securities and Exchange Commission filings for those that are publicly traded. We also review insurance industry developments that may impact the financial condition of our reinsurers. We analyze the credit risk associated with our reinsurance balances recoverable by monitoring the AM Best and Standard & Poor’s (S&P) ratings of our reinsurers. In addition, we subject our reinsurance recoverables to detailed recoverability tests, including a segment-based analysis using the average default rating percentage by S&P rating, which assists the Company in assessing the sufficiency of its allowance. Additionally, we perform an in-depth reinsurer financial condition analysis prior to the renewal of each of our reinsurance placements. ​ Our policy is to charge to earnings, in the form of an allowance, an estimate of unrecoverable amounts from reinsurers. This allowance is reviewed on an ongoing basis to ensure that the amount makes a reasonable provision for reinsurance balances that we may be unable to recover. Once regulatory action (such as receivership, finding of insolvency, order of conservation or order of liquidation) is taken against a reinsurer, the paid and unpaid recoverables for the reinsurer are specifically identified and written off through use of our allowance for estimated unrecoverable amounts from reinsurers. When we write-off such a balance, it is done in full. We then re-evaluate the overall allowance and determine whether the balance is sufficient and, if needed, an additional allowance is recognized. ​ The allowances for uncollectible amounts on paid and unpaid reinsurance recoverables were $19 million and $11 million, respectively, at September 30, 2023. At December 31, 2022, the amounts were $16 million and $11 million, respectively. Changes in the allowances were due to changes in the amount of reinsurance balances outstanding, the composition of reinsurers from whom the balances were recoverable and their associated S&P default ratings. No write-offs or recoveries were applied to the allowances in the first nine months of 2023. We have no receivables with a due date that extends beyond one year that are not included in our allowance for uncollectible amounts. ​ E. INTANGIBLE ASSETS ​ The composition of goodwill and intangible assets at September 30, 2023 and December 31, 2022 is detailed in the following table: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ December 31, (in thousands) 2023 2022 Goodwill ​ ​ ​ ​ ​ ​ Surety ​ $ 40,816 ​ $ 40,816 Casualty ​ ​ 5,246 ​ ​ 5,246 Total goodwill ​ $ 46,062 ​ $ 46,062 Indefinite-lived intangibles ​ ​ 7,500 ​ ​ 7,500 Total goodwill and intangibles ​ $ 53,562 ​ $ 53,562 ​ Annual impairment assessments were performed on our goodwill and state insurance license indefinite-lived intangible assets during the second quarter of 2023. Based upon these reviews, none of the assets were impaired. In addition, there were no triggering events as of September 30, 2023 that would suggest an updated impairment test would be needed for our goodwill and intangible assets. ​ F. EARNINGS PER SHARE ​ Basic earnings per share (EPS) is computed by dividing income available to common shareholders by the weighted-average number of common shares outstanding for the period. Diluted EPS reflects the dilution that could occur if securities or other contracts to issue common stock or common stock equivalents were exercised or converted into common stock. When inclusion of these items increases the earnings per share or reduces the loss per share, the effect on earnings is anti-dilutive. Under these circumstances, the diluted net earnings or net loss per share is computed excluding these items. The following represents a reconciliation of the numerator and denominator of the basic and diluted EPS computations contained in the unaudited condensed consolidated interim financial statements: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Three Months ​ For the Three Months ​ ​ Ended September 30, 2023 ​ Ended September 30, 2022 ​ ​ Income ​ Shares ​ Per Share ​ Income ​ Shares ​ Per Share (in thousands, except per share data) (Numerator) (Denominator) Amount (Numerator) (Denominator) Amount Basic EPS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Earnings available to common shareholders ​ $ 13,536 ​ 45,622 ​ $ 0.30 ​ $ 439,893 ​ 45,379 ​ $ 9.69 Effect of Dilutive Securities ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Stock options and restricted stock units ​ ​ — ​ 443 ​ ​ ​ ​ ​ — ​ 396 ​ ​ ​ Diluted EPS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Earnings available to common shareholders ​ $ 13,536 ​ 46,065 ​ $ 0.29 ​ $ 439,893 ​ 45,775 ​ $ 9.61 Anti-dilutive securities excluded from diluted EPS ​ ​ ​ ​ ​ 145 ​ ​ ​ ​ ​ ​ ​ ​ 435 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Nine Months ​ For the Nine Months ​ ​ Ended September 30, 2023 ​ Ended September 30, 2022 ​ ​ Income ​ Shares ​ Per Share ​ Income ​ Shares ​ Per Share (in thousands, except per share data) (Numerator) (Denominator) Amount (Numerator) (Denominator) Amount Basic EPS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Earnings available to common shareholders ​ $ 189,999 ​ 45,581 ​ $ 4.17 ​ $ 485,577 ​ 45,347 ​ $ 10.71 Effect of Dilutive Securities ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Stock options and restricted stock units ​ ​ — ​ 486 ​ ​ ​ ​ ​ — ​ 428 ​ ​ ​ Diluted EPS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Earnings available to common shareholders ​ $ 189,999 ​ 46,067 ​ $ 4.12 ​ $ 485,577 ​ 45,775 ​ $ 10.61 Anti-dilutive securities excluded from diluted EPS ​ ​ ​ ​ ​ 145 ​ ​ ​ ​ ​ ​ ​ ​ 459 ​ ​ ​ ​ G. COMPREHENSIVE EARNINGS ​ Our comprehensive earnings include net earnings plus after-tax unrealized gains and losses on our available-for-sale fixed income portfolio. In reporting the components of comprehensive earnings, we used the federal statutory tax rate of 21 percent. Other comprehensive earnings (loss), as shown in the consolidated statements of earnings and comprehensive earnings, is net of tax benefit of $15 million for the third quarter of 2023, compared to $22 million for the same period in 2022. For the nine-month period ended September 30, 2023, other comprehensive earnings (loss) is net of tax benefit of $10 million, compared to $78 million for the same period in 2022. ​ Unrealized losses, net of tax, recognized in other comprehensive earnings (loss) were $39 million for the first nine months of 2023, compared to $294 million during the same period last year. The unrealized losses in each period were attributable to an increase in interest rates, which decreased the fair value of securities held in the fixed income portfolio. ​ The following table illustrates the changes in the balance of each component of accumulated other comprehensive earnings (loss) for each period presented in the unaudited condensed consolidated interim financial statements: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (in thousands) ​ For the Three Months ​ For the Nine Months ​ ​ Ended September 30, ​ Ended September 30, Unrealized Gains/Losses on Available-for-Sale Securities 2023 2022 2023 2022 Beginning balance ​ $ (211,090) ​ $ (163,318) ​ $ (229,076) ​ $ 49,826 Other comprehensive earnings (loss) before reclassifications ​ ​ (57,261) ​ ​ (82,785) ​ ​ (41,355) ​ ​ (296,422) Amounts reclassified from accumulated other comprehensive earnings ​ ​ 427 ​ ​ 1,537 ​ ​ 2,507 ​ ​ 2,030 Net current-period other comprehensive earnings (loss) ​ $ (56,834) ​ $ (81,248) ​ $ (38,848) ​ $ (294,392) Ending balance ​ $ (267,924) ​ $ (244,566) ​ $ (267,924) ​ $ (244,566) Balance of securities for which an allowance for credit losses has been recognized in net earnings ​ ​ ​ ​ ​ ​ ​ $ 2,005 ​ $ 1,520 ​ Credit losses on or the sale of an available-for-sale security results in amounts being reclassified from accumulated other comprehensive earnings (loss) to current period net earnings. The effects of reclassifications out of accumulated other comprehensive earnings (loss) by the respective line items of net earnings are presented in the following table: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amount Reclassified from Accumulated Other ​ ​ ​ (in thousands) ​ Comprehensive Earnings (Loss) ​ ​ ​ ​ ​ For the Three Months ​ For the Nine Months ​ ​ ​ Component of Accumulated ​ Ended September 30, ​ Ended September 30, ​ ​ Affected line item in the Other Comprehensive Earnings (Loss) 2023 2022 2023 2022 ​ Statement of Earnings Unrealized gains and losses on available-for-sale securities ​ $ (271) ​ $ (1,912) ​ $ (2,808) ​ $ (2,684) ​ ​ Net realized gains (losses) ​ ​ ​ (270) ​ ​ (34) ​ ​ (365) ​ ​ 115 ​ ​ Credit gains (losses) presented within net realized gains ​ ​ $ (541) ​ $ (1,946) ​ $ (3,173) ​ $ (2,569) ​ ​ Earnings (loss) before income taxes ​ ​ ​ 114 ​ ​ 409 ​ ​ 666 ​ ​ 539 ​ ​ Income tax (expense) benefit ​ ​ $ (427) ​ $ (1,537) ​ $ (2,507) ​ $ (2,030) ​ ​ Net earnings (loss) ​ H. FAIR VALUE MEASUREMENTS ​ Fair value is defined as the price in the principal market that would be received for an asset to facilitate an orderly transaction between market participants on the measurement date. We determined the fair value of certain financial instruments based on their underlying characteristics and relevant transactions in the marketplace. We maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. ​ The following are the levels of the fair value hierarchy and a brief description of the type of valuation inputs that are used to establish each level. Financial assets are classified based upon the lowest level of significant input that is used to determine fair value. ​ Level 1 ​ Level 2 ​ Level 3 ​ As a part of management’s process to determine fair value, we utilize widely recognized, third-party pricing sources to determine our fair values. We have obtained an understanding of the third-party pricing sources’ valuation methodologies and inputs. The following is a description of the valuation techniques used for financial assets that are measured at fair value, including the general classification of such assets pursuant to the fair value hierarchy. ​ Corporate, Agencies, Government and Municipal Bonds: ​ Mortgage-backed Securities (MBS)/Commercial Mortgage-backed Securities (CMBS) and Asset-backed Securities (ABS): ​ Regulation D Private Placement Securities: observable and unobservable inputs. Observable inputs include public corporate spread matrices classified by sector, rating and average life, as well as investment and non-investment grade matrices created from fixed income indices. Unobservable inputs include a liquidity spread premium calculated based on public corporate spread and private corporate spread matrices. The quantitative detail of the liquidity spread premium is neither provided nor reasonably available to the Company. An increase to the credit spread assumptions would result in a lower fair value. ​ For all of our fixed income securities classified as Level 2, we periodically conduct a review to assess the reasonableness of the fair values provided by our pricing services. Our review consists of a two-pronged approach. First, we compare prices provided by our pricing services to those provided by an additional source. In some cases, we obtain prices from securities brokers and compare them to the prices provided by our pricing services. If discrepancies are found in our comparisons, we compare our prices to actual reported trade data for like securities. No changes to the fair values supplied by our pricing services have occurred as a result of our reviews. Based on these assessments, we have determined that the fair values of our Level 2 fixed income securities provided by our pricing services are reasonable. ​ Equity Securities: ​ Due to the relatively short-term nature of cash, short-term investments, accounts receivable and accounts payable, their carrying amounts are reasonable estimates of fair value. Our investments in private funds, classified as other invested assets, are measured using the investments’ net asset value per share and are not categorized within the fair value hierarchy. ​ I. RISKS AND UNCERTAINTIES ​ Certain risks and uncertainties are inherent in our day-to-day operations. Adverse changes in the economy, including inflation; rising interest rates; volatile equity markets; and ongoing supply chain disruptions, could lower demand for our insurance products, result in increased levels of loss costs that we could not anticipate at the time we priced our coverages, or negatively impact our investment results, all of which could have an adverse effect on the profitability of our operations. ​ Catastrophe Exposures ​ Our catastrophe reinsurance treaty renewed on January 1, 2023. We purchased limits of $700 million in excess of $25 million first-dollar retention for earthquakes in California, $700 million in excess of $50 million first-dollar retention for earthquakes outside of California and $600 million in excess of $50 million first-dollar retention for all other perils, including wind. These amounts are subject to certain co-participations by the Company on losses in excess of the first-dollar retentions. On June 1, 2023, we purchased $150 million of additional catastrophe reinsurance protection on top of the previously described coverage. This increases the limits to $850 million in excess of $25 million first-dollar retention for earthquakes in California, $850 million in excess of $50 million first-dollar retention for earthquakes outside of California and $750 million in excess of $50 million first-dollar retention for all other perils, including wind, all of which are still subject to certain co-participations in excess of the retentions.

Investments

Investments	9 Months Ended
	Sep. 30, 2023
Investments Debt And Equity Securities [Abstract]
Investments	2. INVESTMENTS ​ Our investments are primarily composed of fixed income debt securities and common stock equity securities. We carry our equity securities at fair value and categorize all of our debt securities as available-for-sale, which are carried at fair value. ​ Realized gains and losses on disposition of investments are based on the specific identification of the investments sold on the settlement date. The following is a summary of the disposition of fixed income and equity securities for the nine-month periods ended September 30, 2023 and 2022: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Sales ​ Proceeds ​ Gross Realized ​ Net Realized (in thousands) From Sales Gains Losses Gain (Loss) 2023 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fixed income securities - available-for-sale ​ $ 25,983 ​ $ 145 ​ $ (1,035) ​ $ (890) Equity securities ​ ​ 36,708 ​ ​ 15,990 ​ ​ (101) ​ ​ 15,889 2022 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fixed income securities - available-for-sale ​ $ 48,559 ​ $ 286 ​ $ (2,481) ​ $ (2,195) Equity securities ​ ​ 59,264 ​ ​ 20,410 ​ ​ (609) ​ ​ 19,801 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Calls/Maturities ​ ​ ​ ​ Gross Realized ​ Net Realized (in thousands) Proceeds Gains Losses Gain (Loss) 2023 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fixed income securities - available-for-sale ​ $ 434,263 ​ $ 38 ​ $ (236) ​ $ (198) 2022 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fixed income securities - available-for-sale ​ $ 176,991 ​ $ 142 ​ $ (55) ​ $ 87 ​ FAIR VALUE MEASUREMENTS ​ Assets measured at fair value on a recurring basis as of September 30, 2023 and December 31, 2022 are summarized below: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of September 30, 2023 ​ ​ Quoted Prices in ​ Significant Other ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Observable ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Inputs ​ Inputs ​ ​ ​ (in thousands) (Level 1) (Level 2) (Level 3) Total Fixed income securities - available-for-sale ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. government ​ $ — ​ $ 321,611 ​ $ — ​ $ 321,611 U.S. agency ​ ​ — ​ ​ 48,368 ​ ​ — ​ ​ 48,368 Non-U.S. government & agency ​ ​ — ​ ​ 3,811 ​ ​ — ​ ​ 3,811 Agency MBS ​ ​ — ​ ​ 399,381 ​ ​ — ​ ​ 399,381 ABS/CMBS/MBS* ​ ​ — ​ ​ 265,779 ​ ​ — ​ ​ 265,779 Corporate ​ ​ — ​ ​ 1,097,093 ​ ​ 56,291 ​ ​ 1,153,384 Municipal ​ ​ — ​ ​ 525,293 ​ ​ — ​ ​ 525,293 Total fixed income securities - available-for-sale ​ $ — ​ $ 2,661,336 ​ $ 56,291 ​ $ 2,717,627 Equity securities ​ ​ 532,856 ​ ​ — ​ ​ 1,580 ​ ​ 534,436 Total ​ $ 532,856 ​ $ 2,661,336 ​ $ 57,871 ​ $ 3,252,063 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, 2022 ​ ​ Quoted Prices in ​ Significant Other ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Observable ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Inputs ​ Inputs ​ ​ ​ (in thousands) (Level 1) (Level 2) (Level 3) Total Fixed income securities - available-for-sale ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. government ​ $ — ​ $ 454,021 ​ $ — ​ $ 454,021 U.S. agency ​ ​ — ​ ​ 73,063 ​ ​ — ​ ​ 73,063 Non-U.S. government & agency ​ ​ — ​ ​ 5,847 ​ ​ — ​ ​ 5,847 Agency MBS ​ ​ — ​ ​ 331,806 ​ ​ — ​ ​ 331,806 ABS/CMBS/MBS* ​ ​ — ​ ​ 240,736 ​ ​ — ​ ​ 240,736 Corporate ​ ​ — ​ ​ 980,676 ​ ​ 53,654 ​ ​ 1,034,330 Municipal ​ ​ — ​ ​ 527,147 ​ ​ — ​ ​ 527,147 Total fixed income securities - available-for-sale ​ $ — ​ $ 2,613,296 ​ $ 53,654 ​ $ 2,666,950 Equity securities ​ ​ 496,731 ​ ​ 39 ​ ​ 1,612 ​ ​ 498,382 Total ​ $ 496,731 ​ $ 2,613,335 ​ $ 55,266 ​ $ 3,165,332 * Non-agency asset-backed, commercial mortgage-backed and mortgage-backed securities ​ The following table summarizes changes in the balance of securities whose fair value was measured using significant unobservable inputs (Level 3). ​ ​ ​ ​ ​ (in thousands) Level 3 Securities Balance as of January 1, 2023 ​ $ 55,266 Net realized and unrealized gains (losses) ​ ​ ​ Included in other comprehensive earnings (loss) ​ ​ (377) Purchases ​ ​ 5,475 Sales / Calls / Maturities ​ ​ (2,493) Balance as of September 30, 2023 ​ $ 57,871 Change in unrealized gains (losses) during the period for Level 3 assets held at period-end - included in other comprehensive earnings (loss) ​ $ (377) ​ The amortized cost and fair value of available-for-sale fixed income securities by contractual maturity as of September 30, 2023 were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2023 (in thousands) Amortized Cost Fair Value Due in one year or less ​ $ 176,148 ​ $ 174,377 Due after one year through five years ​ ​ 909,286 ​ ​ 858,578 Due after five years through 10 years ​ ​ 653,649 ​ ​ 596,523 Due after 10 years ​ ​ 547,547 ​ ​ 422,989 ABS/CMBS/MBS* ​ ​ 760,155 ​ ​ 665,160 Total available-for-sale ​ $ 3,046,785 ​ $ 2,717,627 * Asset-backed, commercial mortgage-backed and mortgage-backed securities ​ The amortized cost and fair value of available-for-sale securities at September 30, 2023 and December 31, 2022 are presented in the tables below. Amortized cost does not include the $23 million and $20 million of accrued interest receivable as of September 30, 2023 and December 31, 2022, respectively. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2023 ​ ​ Cost or ​ Allowance ​ Gross ​ Gross ​ ​ ​ ​ ​ Amortized ​ for Credit ​ Unrealized ​ Unrealized ​ Fair (in thousands) Cost Losses Gains Losses Value U.S. government ​ $ 334,650 ​ $ — ​ $ — ​ $ (13,039) ​ $ 321,611 U.S. agency ​ ​ 51,477 ​ ​ — ​ ​ — ​ ​ (3,109) ​ ​ 48,368 Non-U.S. government & agency ​ ​ 4,800 ​ ​ — ​ ​ — ​ ​ (989) ​ ​ 3,811 Agency MBS ​ ​ 457,496 ​ ​ — ​ ​ 7 ​ ​ (58,122) ​ ​ 399,381 ABS/CMBS/MBS* ​ ​ 302,659 ​ ​ (4) ​ ​ 65 ​ ​ (36,941) ​ ​ 265,779 Corporate ​ ​ 1,253,152 ​ ​ (700) ​ ​ 952 ​ ​ (100,020) ​ ​ 1,153,384 Municipal ​ ​ 642,551 ​ ​ — ​ ​ 85 ​ ​ (117,343) ​ ​ 525,293 Total Fixed Income ​ $ 3,046,785 ​ $ (704) ​ $ 1,109 ​ $ (329,563) ​ $ 2,717,627 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2022 ​ ​ Cost or ​ Allowance ​ Gross ​ Gross ​ ​ ​ ​ ​ Amortized ​ for Credit ​ Unrealized ​ Unrealized ​ Fair (in thousands) Cost Losses Gains Losses Value U.S. government ​ $ 462,884 ​ $ — ​ $ 8 ​ $ (8,871) ​ $ 454,021 U.S. agency ​ ​ 75,074 ​ ​ — ​ ​ 26 ​ ​ (2,037) ​ ​ 73,063 Non-U.S. government & agency ​ ​ 6,798 ​ ​ — ​ ​ — ​ ​ (951) ​ ​ 5,847 Agency MBS ​ ​ 373,687 ​ ​ — ​ ​ 336 ​ ​ (42,217) ​ ​ 331,806 ABS/CMBS/MBS* ​ ​ 276,126 ​ ​ (8) ​ ​ 62 ​ ​ (35,444) ​ ​ 240,736 Corporate ​ ​ 1,122,097 ​ ​ (331) ​ ​ 541 ​ ​ (87,977) ​ ​ 1,034,330 Municipal ​ ​ 628,607 ​ ​ — ​ ​ 1,265 ​ ​ (102,725) ​ ​ 527,147 Total Fixed Income ​ $ 2,945,273 ​ $ (339) ​ $ 2,238 ​ $ (280,222) ​ $ 2,666,950 * Non-agency asset-backed, commercial mortgage-backed and mortgage-backed securities ​ Allowance for Credit Losses and Unrealized Losses on Fixed Income Securities ​ A reversable allowance for credit losses is recognized on available-for-sale fixed income securities. Several criteria are reviewed to determine if securities in the fixed income portfolio should be included in the allowance for expected credit loss evaluation, including: ​ ● Changes in technology that may impair the earnings potential of the investment, ​ ● The discontinuance of a segment of business that may affect future earnings potential, ​ ● Reduction of or non-payment of interest and/or principal, ​ ● Specific concerns related to the issuer’s industry or geographic area of operation, ​ ● Significant or recurring operating losses, poor cash flows and/or deteriorating liquidity ratios and ​ ● Downgrades in credit quality by a major rating agency. ​ If changes in interest rates and credit spreads do not reasonably explain the unrealized loss for an available-for-sale security or if any of the criteria above indicate a potential credit loss, the security is subjected to a discounted cash flow analysis. Inputs into the discounted cash flow analysis include prepayment assumptions for structured securities, default rates and recoverability rates based on credit rating. The allowance for any security is limited to the amount that the security’s fair value is below amortized cost. As of September 30, 2023, the discounted cash flow analysis resulted in an allowance for credit losses on 19 securities. The following table presents changes in the allowance for expected credit losses on available-for-sale securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, (in thousands) 2023 2022 2023 2022 Beginning balance ​ $ 434 ​ $ 292 ​ $ 339 ​ $ 441 Increase to allowance from securities for which credit losses were not previously recorded ​ ​ 281 ​ ​ 168 ​ ​ 25 ​ ​ 292 Reduction from securities sold during the period ​ ​ — ​ ​ (166) ​ ​ — ​ ​ (619) Reductions from intent to sell securities ​ ​ — ​ ​ — ​ ​ — ​ ​ (17) Net increase (decrease) from securities that had an allowance at the beginning of the period ​ ​ (11) ​ ​ 33 ​ ​ 340 ​ ​ 230 Balance as of September 30, ​ $ 704 ​ $ 327 ​ $ 704 ​ $ 327 ​ During the first nine months of 2023, $2 million of realized losses were recognized on fixed income securities for which the cost basis was written down to fair value due to a credit event, restructurings and losses on securities for which we no longer had the intent to hold until recovery. We recognized less than $1 million of losses on securities for which we no longer had the intent to hold until recovery during the first nine months of 2022. ​ As of September 30, 2023, in addition to the securities included in the allowance for credit losses, the fixed income portfolio contained 1,547 securities with an unrealized loss position for which an allowance for credit losses had not been recorded. The $330 million in associated unrealized losses represents 11 percent of the fixed income portfolio’s cost basis and 10 percent of total invested assets. Isolated to these securities, unrealized losses increased through the first nine months of 2023, as interest rates increased during the period. Of the total 1,547 securities, 1,200 have been in an unrealized loss position for 12 consecutive months or longer. The following table illustrates the total value of fixed income securities that were in an unrealized loss position as of September 30, 2023 and December 31, 2022 after factoring in the allowance for credit losses. All fixed income securities continue to pay the expected coupon payments and we believe we will recover the amortized cost basis of available-for-sale securities that remain in an unrealized loss position. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2023 ​ December 31, 2022 (in thousands) < 12 Mos. 12 Mos. & Greater Total < 12 Mos. 12 Mos. & Greater Total U.S. government ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 82,683 ​ $ 238,928 ​ $ 321,611 ​ $ 399,361 ​ $ 8,828 ​ $ 408,189 Amortized cost ​ ​ 86,186 ​ ​ 248,464 ​ ​ 334,650 ​ ​ 407,340 ​ ​ 9,720 ​ ​ 417,060 Unrealized loss ​ $ (3,503) ​ $ (9,536) ​ $ (13,039) ​ $ (7,979) ​ $ (892) ​ $ (8,871) U.S. agency ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 15,658 ​ $ 32,710 ​ $ 48,368 ​ $ 32,987 ​ $ 2,170 ​ $ 35,157 Amortized cost ​ ​ 16,304 ​ ​ 35,173 ​ ​ 51,477 ​ ​ 34,627 ​ ​ 2,567 ​ ​ 37,194 Unrealized loss ​ $ (646) ​ $ (2,463) ​ $ (3,109) ​ $ (1,640) ​ $ (397) ​ $ (2,037) Non-U.S. government ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ — ​ $ 3,811 ​ $ 3,811 ​ $ 3,626 ​ $ 2,221 ​ $ 5,847 Amortized cost ​ ​ — ​ ​ 4,800 ​ ​ 4,800 ​ ​ 3,798 ​ ​ 3,000 ​ ​ 6,798 Unrealized Loss ​ $ — ​ $ (989) ​ $ (989) ​ $ (172) ​ $ (779) ​ $ (951) Agency MBS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 138,173 ​ $ 260,503 ​ $ 398,676 ​ $ 197,252 ​ $ 117,851 ​ $ 315,103 Amortized cost ​ ​ 143,662 ​ ​ 313,136 ​ ​ 456,798 ​ ​ 212,776 ​ ​ 144,544 ​ ​ 357,320 Unrealized loss ​ $ (5,489) ​ $ (52,633) ​ $ (58,122) ​ $ (15,524) ​ $ (26,693) ​ $ (42,217) ABS/CMBS/MBS* ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 41,219 ​ $ 217,289 ​ $ 258,508 ​ $ 96,754 ​ $ 136,149 ​ $ 232,903 Amortized cost ​ ​ 41,882 ​ ​ 253,567 ​ ​ 295,449 ​ ​ 104,724 ​ ​ 163,623 ​ ​ 268,347 Unrealized loss ​ $ (663) ​ $ (36,278) ​ $ (36,941) ​ $ (7,970) ​ $ (27,474) ​ $ (35,444) Corporate ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 274,290 ​ $ 804,638 ​ $ 1,078,928 ​ $ 660,830 ​ $ 323,337 ​ $ 984,167 Amortized cost ​ ​ 287,456 ​ ​ 891,492 ​ ​ 1,178,948 ​ ​ 697,437 ​ ​ 374,707 ​ ​ 1,072,144 Unrealized loss ​ $ (13,166) ​ $ (86,854) ​ $ (100,020) ​ $ (36,607) ​ $ (51,370) ​ $ (87,977) Municipal ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 154,928 ​ $ 358,021 ​ $ 512,949 ​ $ 228,827 ​ $ 204,324 ​ $ 433,151 Amortized cost ​ ​ 159,582 ​ ​ 470,710 ​ ​ 630,292 ​ ​ 255,240 ​ ​ 280,636 ​ ​ 535,876 Unrealized loss ​ $ (4,654) ​ $ (112,689) ​ $ (117,343) ​ $ (26,413) ​ $ (76,312) ​ $ (102,725) Total fixed income ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 706,951 ​ $ 1,915,900 ​ $ 2,622,851 ​ $ 1,619,637 ​ $ 794,880 ​ $ 2,414,517 Amortized cost ​ ​ 735,072 ​ ​ 2,217,342 ​ ​ 2,952,414 ​ ​ 1,715,942 ​ ​ 978,797 ​ ​ 2,694,739 Unrealized loss ​ $ (28,121) ​ $ (301,442) ​ $ (329,563) ​ $ (96,305) ​ $ (183,917) ​ $ (280,222) * Non-agency asset-backed, commercial mortgage-backed and mortgage-backed securities ​ The following table shows the composition of the fixed income securities in unrealized loss positions, after factoring in the allowance for credit losses, at September 30, 2023 by the National Association of Insurance Commissioners (NAIC) rating and the generally equivalent Standard & Poor’s (S&P) and Moody’s ratings. The vast majority of the securities are rated by S&P and/or Moody’s. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Equivalent ​ Equivalent ​ (dollars in thousands) ​ ​ ​ NAIC S&P Moody’s ​ Amortized ​ ​ ​ ​ Unrealized ​ Percent ​ Rating Rating Rating Cost Fair Value Loss to Total ​ 1 ​ AAA/AA/A ​ Aaa/Aa/A ​ $ 2,440,094 ​ $ 2,160,142 ​ $ (279,952) ​ 85.0 % 2 ​ BBB ​ Baa ​ ​ 431,695 ​ ​ 388,396 ​ ​ (43,299) ​ 13.1 % 3 ​ BB ​ Ba ​ ​ 48,627 ​ ​ 44,282 ​ ​ (4,345) ​ 1.3 % 4 ​ B ​ B ​ ​ 29,382 ​ ​ 28,045 ​ ​ (1,337) ​ 0.4 % 5 ​ CCC ​ Caa ​ ​ 2,616 ​ ​ 1,986 ​ ​ (630) ​ 0.2 % 6 ​ CC or lower ​ Ca or lower ​ ​ — ​ ​ — ​ ​ — ​ 0.0 % ​ ​ ​ ​ Total ​ $ 2,952,414 ​ $ 2,622,851 ​ $ (329,563) ​ 100.0 % ​ Other Invested Assets ​ We had $60 million of other invested assets at September 30, 2023, compared to $48 million at December 31, 2022. Other invested assets include investments in low income housing tax credit partnerships (LIHTC) and historic tax credit partnerships (HTC), membership in the Federal Home Loan Bank of Chicago (FHLBC), and investments in private funds. Our LIHTC and HTC investments are carried at amortized cost and our investment in FHLBC stock is carried at cost. Due to the nature of the LIHTC, HTC and our membership in the FHLBC, their carrying amounts approximate fair value. The private funds are carried at fair value, using each investment’s net asset value. ​ Our LIHTC interests had a balance of $11 million at September 30, 2023, compared to $13 million on December 31, 2022. Our LIHTC interests recognized amortization of $0.8 million as a component of income tax expense and a total tax benefit of $0.8 million during the third quarter of 2023, compared to $0.8 million of amortization and $0.9 million of tax benefit during the same period in 2022. For the nine-months ended September 30, 2023, our LIHTC interest recognized amortization of $2.3 million and a total benefit of $2.4 million, compared to $2.5 million of amortization and $2.6 million of tax benefit for the same period in 2022. Our unfunded commitment for our LIHTC investments was less than $1 million at September 30, 2023 and will be paid out in installments through 2035. ​ Our HTC investment had a balance of $14 million at September 30, 2023, compared to $11 million at December 31, 2022. Through 2022, the investment was accounted for as an investment in unconsolidated investee. Due to the adoption of ASU 2023-02, Accounting for Investments in Tax Credit Structures Using the Proportional Amortization Method ​ As of September 30, 2023, $57 million of investments were pledged as collateral with the FHLBC to ensure timely access to the secured lending facility that ownership of FHLBC stock provides. As of September 30, 2023, $50 million of borrowings were outstanding with the FHLBC. ​ Our investments in private funds totaled $27 million as of September 30, 2023, down from $28 million as of December 31, 2022, and had $5 million of associated unfunded commitments at September 30, 2023. Our interest in private funds is generally restricted from being transferred or otherwise redeemed without prior consent by the respective entities, and the timed dissolution of the partnerships would trigger redemption. ​ Investments in Unconsolidated Investees ​ We had $56 million of investments in unconsolidated investees at September 30, 2023, compared to $58 million at December 31, 2022. At September 30, 2023, our investment in Prime Holdings Insurance Services, Inc. (Prime) was $56 million and other investments in unconsolidated investees totaled less than $1 million. Through December 31, 2022, our $11 million HTC investment was accounted for as an unconsolidated investee, but was reclassified as an other invested asset during 2023 due to the adoption of ASU 2023-02. ​ Cash and Short-Term Investments ​ Cash consists of uninvested balances in bank accounts. Short-term investments consist of investments with original maturities of 90 days or less, primarily AAA-rated government money market funds. Short-term investments are carried at cost. We had a cash and short-term investment balance of $18 million and $125 million, respectively, at September 30, 2023, compared to $23 million and $36 million, respectively, at December 31, 2022.

Debt

Debt	9 Months Ended
	Sep. 30, 2023
Debt Disclosure [Abstract]
Debt	3. DEBT ​ As of September 30, 2023, we had $100 million in debt outstanding. On September 15, 2023, we retired $150 million in senior notes that were originally issued in 2013. Additionally, on September 15, 2023, we accessed $50 million from our revolving line of credit with PNC Bank, N.A. (PNC). The borrowing may be repaid at any time and carries a floating interest rate of 7.07 percent which will reset during the first quarter of 2024. The credit facility with PNC was entered into during the first quarter of 2023 and replaced the previous $60 million facility with Bank of Montreal, Chicago Branch, which expired on March 27, 2023. The line of credit permits us to borrow up to an aggregate principal amount of $100 million, but may be increased up to an aggregate principal amount of $130 million under certain conditions. The facility has a three-year term that expires on May 29, 2026. Further, RLI Insurance Company borrowed $50 million from the Federal Home Loan Bank of Chicago (FHLBC) on November 10, 2021. The borrowing matures on November 10, 2023 and has an option to be paid off prior to maturity. Interest is paid monthly at an annualized rate of 0.84 percent. ​

Historical Loss And LAE Develop

Historical Loss And LAE Development	9 Months Ended
	Sep. 30, 2023
Historical Loss And L A E Development Disclosure [Abstract]
Historical Loss And LAE Development	4. HISTORICAL LOSS AND LAE DEVELOPMENT ​ The following table is a reconciliation of our unpaid losses and settlement expenses (LAE) for the first nine months of 2023 and 2022: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Nine Months ​ ​ Ended September 30, (in thousands) 2023 2022 Unpaid losses and LAE at beginning of year ​ ​ ​ ​ ​ ​ Gross ​ $ 2,315,637 ​ $ 2,043,555 Ceded ​ ​ (740,089) ​ ​ (608,086) Net ​ $ 1,575,548 ​ $ 1,435,469 ​ ​ ​ ​ ​ ​ ​ Increase (decrease) in incurred losses and LAE ​ ​ ​ ​ ​ ​ Current accident year ​ $ 551,975 ​ $ 491,559 Prior accident years ​ ​ (93,986) ​ ​ (103,032) Total incurred ​ $ 457,989 ​ $ 388,527 ​ ​ ​ ​ ​ ​ ​ Loss and LAE payments for claims incurred ​ ​ ​ ​ ​ ​ Current accident year ​ $ (113,010) ​ $ (52,435) Prior accident years ​ ​ (252,786) ​ ​ (219,682) Total paid ​ $ (365,796) ​ $ (272,117) ​ ​ ​ ​ ​ ​ ​ Net unpaid losses and LAE at September 30, ​ $ 1,667,741 ​ $ 1,551,879 ​ ​ ​ ​ ​ ​ ​ Unpaid losses and LAE at September 30, ​ ​ ​ ​ ​ ​ Gross ​ $ 2,488,695 ​ $ 2,322,737 Ceded ​ ​ (820,954) ​ ​ (770,858) Net ​ $ 1,667,741 ​ $ 1,551,879 ​ For the first nine months of 2023, incurred losses and LAE included $94 million of favorable development on prior years’ loss reserves, largely from accident years 2016 through 2018 and 2020 through 2022. Commercial excess, executive products, general liability, professional services, personal umbrella, surety and commercial property were drivers of the favorable development. No products experienced significant adverse development. ​ For the first nine months of 2022, incurred losses and LAE included $103 million of favorable development on prior years’ loss reserves, largely from accident years 2018 through 2021. General liability, professional services, transportation, executive products, commercial excess, marine and surety were drivers of the favorable development. No products experienced significant adverse development.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2023
Income Tax Disclosure [Abstract]
Income Taxes	5. INCOME TAXES ​ Our effective tax rate for the three and nine months ended September 30, 2023 was 9.9 percent and 18.7 percent, respectively, compared to 20.7 percent and 20.0 percent, respectively, for the same period in 2022. Effective rates are dependent upon components of pretax earnings and the related tax effects. The effective tax rate was lower for the three and nine-month periods in 2023, as lower pretax income increased the percentage impact of tax-favored adjustments. ​ Income tax expense attributable to income from operations for the three and nine-month period ended September 30, 2023 and 2022 differed from the amounts computed by applying the U.S. federal tax rate of 21 percent to pretax income by the items detailed in the below table. In interim periods, income taxes are adjusted to reflect the effective tax rate we anticipate for the year, with adjustments flowing through the other items, net line. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Three Months Ended September 30, 2023 ​ For the Nine Months Ended September 30, 2023 ​ ​ ​ 2023 ​ 2022 ​ 2023 ​ 2022 ​ (in thousands) Amount % Amount % Amount % Amount % ​ Provision for income taxes at the statutory rate of 21% ​ $ 3,154 ​ 21.0 % $ 116,487 ​ 21.0 % $ 49,107 ​ 21.0 % $ 127,401 ​ 21.0 % Increase (reduction) in taxes resulting from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Excess tax benefit on share-based compensation ​ ​ (395) ​ (2.6) % ​ (95) ​ (0.0) % ​ (3,263) ​ (1.4) % ​ (2,270) ​ (0.4) % Tax exempt interest income ​ ​ (270) ​ (1.8) % ​ (280) ​ (0.1) % ​ (827) ​ (0.4) % ​ (850) ​ (0.1) % Dividends received deduction ​ ​ (164) ​ (1.1) % ​ (240) ​ (0.0) % ​ (597) ​ (0.3) % ​ (673) ​ (0.1) % Tax credit ​ ​ (779) ​ (5.2) % ​ (1,461) ​ (0.3) % ​ (2,338) ​ (1.0) % ​ (4,383) ​ (0.7) % ESOP dividends paid deduction ​ ​ (142) ​ (1.0) % ​ (136) ​ (0.0) % ​ (423) ​ (0.2) % ​ (404) ​ (0.0) % Nondeductible expenses ​ ​ 158 ​ 1.1 % ​ 34 ​ 0.0 % ​ 1,505 ​ 0.6 % ​ 608 ​ 0.1 % Other items, net ​ ​ (79) ​ (0.5) % ​ 500 ​ 0.1 % ​ 678 ​ 0.4 % ​ 1,667 ​ 0.2 % Total tax expense ​ $ 1,483 ​ 9.9 % $ 114,809 ​ 20.7 % $ 43,842 ​ 18.7 % $ 121,096 ​ 20.0 % ​ We have recorded our deferred tax assets and liabilities using the statutory federal tax rate of 21 percent. We believe it is more likely than not that all deferred tax assets will be recovered, given the carry back availability as well as the result of future operations, which will generate sufficient taxable income to realize the deferred tax asset.

Stock Based Compensation

Stock Based Compensation	9 Months Ended
	Sep. 30, 2023
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Based Compensation	6. STOCK BASED COMPENSATION ​ Our RLI Corp. Long-Term Incentive Plan (2015 LTIP) was in place from 2015 to 2023. The 2015 LTIP provided for equity-based compensation, including stock options and restricted stock units, up to a maximum of 4,000,000 shares of common stock (subject to adjustment for changes in our capitalization and other events). Between 2015 and 2023, we granted 3,291,388 awards under the 2015 LTIP. The 2015 LTIP was replaced in 2023. ​ In 2023, our shareholders approved the 2023 RLI Corp. Long-Term Incentive Plan (2023, LTIP), which provides for equity-based compensation. In conjunction with the adoption of the 2023 LTIP, effective May 4, 2023, awards are no longer granted under the 2015 LTIP. Awards under the 2023 LTIP may be in the form of restricted stock, restricted stock units, stock options (incentive or non-qualified), stock appreciation rights, performance units as well as other stock-based awards. Eligibility under the 2023 LTIP is limited to employees, directors, consultants and independent contractors of the Company or any affiliate. The granting of awards under the 2023 LTIP is solely at the discretion of the Human Capital and Compensation Committee of the board of directors or its delegate. The maximum number of shares of common stock available for distribution under the 2023 LTIP is 4,004,891 shares (subject to adjustment for changes in our capitalization and other events). Since the plan’s approval in 2023, we have granted 165,063 awards under the 2023 LTIP. ​ Compensation expense is based on the probable number of awards expected to vest. The total compensation expense related to equity awards was $2.0 million and $6.6 million in the three and nine-month periods ended September 30, 2023, respectively, compared to $2.4 million and $6.2 million, respectively, for the same period in 2022. The total income tax benefit was $0.3 million and $1.1 million for the three and nine-month periods ended September 30, 2023, compared to $0.4 million and $1.0 million, respectively, for the same periods in 2022. Total unrecognized compensation expense relating to outstanding and unvested awards was $7 million, which will be recognized over the weighted average vesting period of 2.97 years. ​ Stock Options ​ Under the 2023 LTIP, as under the 2015 LTIP, we grant stock options for shares with an exercise price equal to the fair market value of the shares at the date of grant (subject to adjustments for changes in our capitalization, special dividends and other events as set forth in such plans). Options generally vest and become exercisable over a five-year period and expire eight years after grant. ​ For most participants, the requisite service period and vesting period will be the same. For participants who are retirement eligible, defined by the plan as those individuals whose age and years of service equals 75 or greater, the requisite service period is deemed to be met and options are immediately expensed on the date of grant. For participants who will become retirement eligible during the vesting period, the requisite service period over which expense is recognized is the period between the grant date and the attainment of retirement eligibility. Shares issued upon option exercise are newly issued shares. ​ The following tables summarize option activity for the nine-month period ended September 30, 2023: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ Aggregate ​ ​ ​ ​ Weighted ​ Average ​ Intrinsic ​ ​ ​ ​ Average ​ Remaining ​ Value ​ Options Exercise Price Contractual Life (in 000’s) Outstanding options at January 1, 2023 ​ 1,695,660 ​ $ 82.42 ​ ​ ​ ​ ​ Options granted ​ 172,250 ​ ​ 135.51 ​ ​ ​ ​ ​ Options exercised ​ (219,905) ​ ​ 57.15 ​ ​ ​ ​ ​ Options canceled/forfeited ​ (8,585) ​ ​ 96.99 ​ ​ ​ ​ ​ Outstanding options at September 30, 2023 ​ 1,639,420 ​ $ 91.31 ​ 4.83 ​ $ 73,168 Exercisable options at September 30, 2023 ​ 849,997 ​ $ 76.60 ​ 3.64 ​ $ 50,400 ​ The intrinsic value, which is the difference between the fair value and the exercise price, of options exercised was $17 million and $6 million during the first nine months of 2023 and 2022, respectively. ​ The fair value of options was estimated using a Black-Scholes based option pricing model with the following weighted average grant-date assumptions and weighted average fair values as of September 30: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2023 2022 ​ Weighted-average fair value of grants ​ $ 26.71 ​ $ 20.99 ​ Risk-free interest rates ​ ​ 3.41 % ​ 2.86 % Dividend yield ​ ​ 2.29 % ​ 2.50 % Expected volatility ​ ​ 22.96 % ​ 22.89 % Expected option life ​ ​ 4.94 years ​ 5.06 years ​ The risk-free rate was determined based on U.S. treasury yields that most closely approximated the options’ expected life. The dividend yield was determined based on the average annualized quarterly dividends paid during the most recent five-year period and incorporated a consideration for special dividends paid in recent history. The expected volatility was calculated based on the median of the rolling volatilities for the expected life of the options. The expected option life was determined based on historical exercise behavior and the assumption that all outstanding options will be exercised at the midpoint of the current date and remaining contractual term, adjusted for the demographics of the current year’s grant. ​ Restricted Stock Units ​ In addition to stock options, restricted stock units (RSUs) are granted with a value equal to the closing stock price of the Company’s stock on the dates the units are granted. For employees, these units generally have a three-year cliff vesting, but have an accelerated vesting feature for participants who are retirement eligible, defined by the plan as those individuals whose age and years of service equals 75 or greater. For directors, these units vest on the earlier of one year from the date of grant or the next annual shareholders meeting. In addition, the RSUs have dividend participation, which accrue as additional units and are settled with granted stock units at the end of the vesting period. The total fair value of restricted stock units that vested was $3 million and $2 million during the first nine months of 2023 and 2022, respectively. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ Average ​ ​ ​ ​ Grant Date ​ RSUs Fair Value Nonvested at January 1, 2023 ​ 44,208 ​ $ 109.51 Granted ​ 20,413 ​ ​ 136.19 Reinvested ​ 288 ​ ​ 132.60 Vested ​ (19,957) ​ ​ 103.61 Forfeited ​ (1,795) ​ ​ 116.10 Nonvested at September 30, 2023 ​ 43,157 ​ $ 124.74 ​

Operating Segment Information

Operating Segment Information	9 Months Ended
	Sep. 30, 2023
Segment Reporting [Abstract]
Operating Segment Information	7. OPERATING SEGMENT INFORMATION Selected information by operating segment is presented in the table below. Additionally, the table reconciles segment totals to total earnings and total revenues. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Three Months ​ For the Nine Months Revenues ​ Ended September 30, ​ Ended September 30, (in thousands) 2023 2022 2023 2022 Casualty ​ $ 189,305 ​ $ 179,615 ​ $ 562,384 ​ $ 528,494 Property ​ ​ 94,988 ​ ​ 80,844 ​ ​ 285,596 ​ ​ 222,974 Surety ​ ​ 34,116 ​ ​ 31,009 ​ ​ 100,432 ​ ​ 91,962 Net premiums earned ​ $ 318,409 ​ $ 291,468 ​ $ 948,412 ​ $ 843,430 Net investment income ​ ​ 31,963 ​ ​ 21,270 ​ ​ 87,835 ​ ​ 57,625 Net realized gains ​ ​ 6,558 ​ ​ 573,170 ​ ​ 26,758 ​ ​ 591,562 Net unrealized gains (losses) on equity securities ​ ​ (25,236) ​ ​ (26,414) ​ ​ 15,474 ​ ​ (155,218) Total consolidated revenue ​ $ 331,694 ​ $ 859,494 ​ $ 1,078,479 ​ $ 1,337,399 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net Earnings ​ ​ ​ ​ ​ ​ (in thousands) 2023 2022 2023 2022 Casualty ​ $ 18,670 ​ $ 11,329 ​ $ 57,478 ​ $ 60,418 Property ​ ​ (20,671) ​ ​ (8,308) ​ ​ 33,589 ​ ​ 40,273 Surety ​ ​ 6,248 ​ ​ 5,775 ​ ​ 22,271 ​ ​ 23,539 Net underwriting income (loss) ​ $ 4,247 ​ $ 8,796 ​ $ 113,338 ​ $ 124,230 Net investment income ​ ​ 31,963 ​ ​ 21,270 ​ ​ 87,835 ​ ​ 57,625 Net realized gains ​ ​ 6,558 ​ ​ 573,170 ​ ​ 26,758 ​ ​ 591,562 Net unrealized gains (losses) on equity securities ​ ​ (25,236) ​ ​ (26,414) ​ ​ 15,474 ​ ​ (155,218) General corporate expense and interest on debt ​ ​ (4,245) ​ ​ (4,768) ​ ​ (16,733) ​ ​ (14,587) Equity in earnings (loss) of unconsolidated investees ​ ​ 1,732 ​ ​ (17,352) ​ ​ 7,169 ​ ​ 3,061 Earnings before income taxes ​ $ 15,019 ​ $ 554,702 ​ $ 233,841 ​ $ 606,673 Income tax expense ​ ​ 1,483 ​ ​ 114,809 ​ ​ 43,842 ​ ​ 121,096 Net earnings ​ $ 13,536 ​ $ 439,893 ​ $ 189,999 ​ $ 485,577 ​ The following table further summarizes revenues by major product type within each operating segment: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Three Months ​ For the Nine Months Net Premiums Earned ​ Ended September 30, ​ Ended September 30, (in thousands) 2023 2022 2023 2022 Casualty ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Commercial excess and personal umbrella ​ $ 72,559 ​ $ 65,131 ​ $ 209,402 ​ $ 187,666 General liability ​ ​ 25,250 ​ ​ 25,284 ​ ​ 77,166 ​ ​ 74,130 Commercial transportation ​ ​ 25,843 ​ ​ 24,205 ​ ​ 76,473 ​ ​ 71,774 Professional services ​ ​ 24,882 ​ ​ 24,174 ​ ​ 73,841 ​ ​ 71,590 Small commercial ​ ​ 18,096 ​ ​ 17,005 ​ ​ 54,492 ​ ​ 50,463 Executive products ​ ​ 5,943 ​ ​ 6,556 ​ ​ 18,449 ​ ​ 20,208 Other casualty ​ ​ 16,732 ​ ​ 17,260 ​ ​ 52,561 ​ ​ 52,663 Total ​ $ 189,305 ​ $ 179,615 ​ $ 562,384 ​ $ 528,494 Property ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Commercial property ​ $ 63,783 ​ $ 43,201 ​ $ 173,264 ​ $ 115,419 Marine ​ ​ 33,436 ​ ​ 29,592 ​ ​ 96,484 ​ ​ 84,499 Other property ​ ​ (2,231) ​ ​ 8,051 ​ ​ 15,848 ​ ​ 23,056 Total ​ $ 94,988 ​ $ 80,844 ​ $ 285,596 ​ $ 222,974 Surety ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Commercial ​ $ 12,653 ​ $ 12,057 ​ $ 37,576 ​ $ 35,423 Miscellaneous ​ ​ 11,966 ​ ​ 10,751 ​ ​ 35,900 ​ ​ 33,691 Contract ​ ​ 9,497 ​ ​ 8,201 ​ ​ 26,956 ​ ​ 22,848 Total ​ $ 34,116 ​ $ 31,009 ​ $ 100,432 ​ $ 91,962 Grand Total ​ $ 318,409 ​ $ 291,468 ​ $ 948,412 ​ $ 843,430 ​

Leases

Leases	9 Months Ended
	Sep. 30, 2023
Leases [Abstract]
Leases	8. LEASES ​ Right-of-use (ROU) assets are included in the other assets line item and lease liabilities are included in the other liabilities line item of the consolidated balance sheet. We determine if a contract contains a lease at inception and recognize operating lease ROU assets and operating lease liabilities based on the present value of the future minimum lease payments at the commencement date. As our leases do not provide an implicit rate, we use our incremental borrowing rate based on the information available at the commencement date in determining the present value of future payments. Lease agreements may include options to extend or terminate. The options are exercised at our discretion and are included in operating lease liabilities if it is reasonably certain the option will be exercised. Lease agreements have lease and non-lease components, which are accounted for as a single lease component. Operating lease costs for future minimum lease payments are recognized on a straight-line basis over the lease terms. Variable lease costs are expensed in the period in which the obligations are incurred. Sublease income is recognized on a straight-line basis over the sublease term. ​ The Company’s operating lease obligations are for branch office facilities. The components of lease expense and other lease information as of and during the three and nine-month periods ended September 30, 2023 and 2022 were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Three Months ​ For the Nine Months ​ ​ Ended September 30, ​ Ended September 30, (in thousands) 2023 2022 2023 2022 Operating lease cost ​ $ 1,235 ​ $ 1,272 ​ $ 3,743 ​ $ 3,686 Variable lease cost ​ ​ 320 ​ ​ 339 ​ ​ 1,202 ​ ​ 1,073 Sublease income ​ ​ (139) ​ ​ (139) ​ ​ (416) ​ ​ (416) Total lease cost ​ $ 1,416 ​ $ 1,472 ​ $ 4,529 ​ $ 4,343 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cash paid for amounts included in measurement of lease liabilities ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Operating cash outflows from operating leases ​ $ 1,431 ​ $ 1,387 ​ $ 4,186 ​ $ 3,987 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ROU assets obtained in exchange for new operating lease liabilities ​ $ 319 ​ $ 54 ​ $ 1,303 ​ $ 2,485 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Reduction to ROU assets resulting from reduction to lease liabilities ​ $ — ​ $ — ​ $ 200 ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Other non-cash reductions to ROU assets ​ $ — ​ $ — ​ $ — ​ $ 73 ​ ​ ​ ​ ​ ​ ​ ​ ​ (in thousands) September 30, 2023 December 31, 2022 ​ Operating lease ROU assets ​ $ 10,339 ​ $ 12,766 ​ Operating lease liabilities ​ $ 11,593 ​ $ 14,499 ​ Weighted-average remaining lease term - operating leases ​ ​ 4.16 years ​ 4.21 years Weighted-average discount rate - operating leases ​ ​ 2.06 % ​ 2.11 % ​ Future minimum lease payments under non-cancellable leases as of September 30, 2023 were as follows: ​ ​ ​ ​ ​ (in thousands) September 30, 2023 2023 ​ $ 1,205 2024 ​ ​ 3,919 2025 ​ ​ 2,709 2026 ​ ​ 1,744 2027 ​ ​ 997 2028 ​ ​ 704 Thereafter ​ ​ 884 Total future minimum lease payments ​ $ 12,162 Less imputed interest ​ ​ (569) Total operating lease liability ​ $ 11,593 ​

Acquisitions and Dispositions

Acquisitions and Dispositions	9 Months Ended
	Sep. 30, 2023
Business Combination and Asset Acquisition [Abstract]
Acquisitions and Dispositions	9. ACQUISITONS AND DISPOSTIONS ​ On September 30, 2022, RLI Corp. completed the sale of its equity method investment in Maui Jim, Inc. to Kering Eyewear for cash proceeds of $687 million. A net realized gain of $571 million was recognized during 2022, and the payout of the working capital escrow during the first quarter of 2023 resulted in the recognition of an additional $14 million realized gain. The gains were recorded in the net realized gain line item of the statement of earnings.

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2023
Accounting Policies [Abstract]
Basis of Presentation	A. BASIS OF PRESENTATION ​ The unaudited condensed consolidated interim financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (GAAP) for interim financial reporting and with the instructions to Form 10-Q and Regulation S-X. Accordingly, they do not include all the disclosures required by GAAP for complete financial statements. As such, these unaudited condensed consolidated interim financial statements should be read in conjunction with our 2022 Annual Report on Form 10-K. Management believes that the disclosures are adequate to make the information presented not misleading, and all normal and recurring adjustments necessary to present fairly the financial position at September 30, 2023 and the results of operations of RLI Corp. (the Company) and subsidiaries for all periods presented have been made. The results of operations for any interim period are not necessarily indicative of the operating results for a full year. Certain reclassifications were made to 2022 to conform to the classifications used in the current year. ​ The preparation of the unaudited condensed consolidated interim financial statements requires management to make estimates and assumptions relating to the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the unaudited condensed consolidated interim financial statements and the reported amounts of revenue and expenses during the period. These estimates are inherently subject to change and actual results could differ significantly from these estimates.
Adopted Accounting Standards	B. ADOPTED ACCOUNTING STANDARDS ​ 2023-02—Investments – Equity Method and Joint Ventures (Topic 323): Accounting for Investments in Tax Credit Structures Using the Proportional Method ​ The amendments in this ASU permit the use of the proportional amortization method for investments in tax credits if certain conditions are met. Under the proportional amortization method, the initial cost of an investment is amortized in proportion to the amount of tax credits and other tax benefits received, with the amortization and tax credits presented as a component of income tax expense. Under previous guidance, equity investments in tax credit structures, other than qualified affordable housing projects, were accounted for using the equity method of accounting, which required the presentation of income, gains and losses, and tax credits in their respective line items of the statement of earnings. This ASU allows entities to make an accounting policy election on an individual tax credit program basis for all equity investments whose primary purpose is receiving income tax credits or other income tax benefits. When the proportional amortization method is selected, this amendment also requires a liability be recognized for delayed equity contributions that are unconditional or for contingent contributions when the contingent event becomes probable. ​ We adopted ASU 2023-02
Prospective Accounting Standards	​ C. PROSPECTIVE ACCOUNTING STANDARDS ​ There are no prospective accounting standards which would have a material impact on our financial statements as of September 30, 2023.
Reinsurance	​ D. REINSURANCE ​ Ceded unearned premiums and reinsurance balances recoverable on unpaid losses and settlement expenses are reported separately as an asset, rather than being netted with the related liability, since reinsurance does not relieve the Company of our liability to policyholders. Such balances are subject to the credit risk associated with the individual reinsurer. We continually monitor the financial condition of our reinsurers and actively follow up on any past due or disputed amounts. As part of our monitoring efforts, we review reinsurers’ annual financial statements and Securities and Exchange Commission filings for those that are publicly traded. We also review insurance industry developments that may impact the financial condition of our reinsurers. We analyze the credit risk associated with our reinsurance balances recoverable by monitoring the AM Best and Standard & Poor’s (S&P) ratings of our reinsurers. In addition, we subject our reinsurance recoverables to detailed recoverability tests, including a segment-based analysis using the average default rating percentage by S&P rating, which assists the Company in assessing the sufficiency of its allowance. Additionally, we perform an in-depth reinsurer financial condition analysis prior to the renewal of each of our reinsurance placements. ​ Our policy is to charge to earnings, in the form of an allowance, an estimate of unrecoverable amounts from reinsurers. This allowance is reviewed on an ongoing basis to ensure that the amount makes a reasonable provision for reinsurance balances that we may be unable to recover. Once regulatory action (such as receivership, finding of insolvency, order of conservation or order of liquidation) is taken against a reinsurer, the paid and unpaid recoverables for the reinsurer are specifically identified and written off through use of our allowance for estimated unrecoverable amounts from reinsurers. When we write-off such a balance, it is done in full. We then re-evaluate the overall allowance and determine whether the balance is sufficient and, if needed, an additional allowance is recognized. ​ The allowances for uncollectible amounts on paid and unpaid reinsurance recoverables were $19 million and $11 million, respectively, at September 30, 2023. At December 31, 2022, the amounts were $16 million and $11 million, respectively. Changes in the allowances were due to changes in the amount of reinsurance balances outstanding, the composition of reinsurers from whom the balances were recoverable and their associated S&P default ratings. No write-offs or recoveries were applied to the allowances in the first nine months of 2023. We have no receivables with a due date that extends beyond one year that are not included in our allowance for uncollectible amounts. ​
Goodwill and Intangible Assets	E. INTANGIBLE ASSETS ​ The composition of goodwill and intangible assets at September 30, 2023 and December 31, 2022 is detailed in the following table: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ December 31, (in thousands) 2023 2022 Goodwill ​ ​ ​ ​ ​ ​ Surety ​ $ 40,816 ​ $ 40,816 Casualty ​ ​ 5,246 ​ ​ 5,246 Total goodwill ​ $ 46,062 ​ $ 46,062 Indefinite-lived intangibles ​ ​ 7,500 ​ ​ 7,500 Total goodwill and intangibles ​ $ 53,562 ​ $ 53,562 ​ Annual impairment assessments were performed on our goodwill and state insurance license indefinite-lived intangible assets during the second quarter of 2023. Based upon these reviews, none of the assets were impaired. In addition, there were no triggering events as of September 30, 2023 that would suggest an updated impairment test would be needed for our goodwill and intangible assets.
Earnings Per Share	F. EARNINGS PER SHARE ​ Basic earnings per share (EPS) is computed by dividing income available to common shareholders by the weighted-average number of common shares outstanding for the period. Diluted EPS reflects the dilution that could occur if securities or other contracts to issue common stock or common stock equivalents were exercised or converted into common stock. When inclusion of these items increases the earnings per share or reduces the loss per share, the effect on earnings is anti-dilutive. Under these circumstances, the diluted net earnings or net loss per share is computed excluding these items. The following represents a reconciliation of the numerator and denominator of the basic and diluted EPS computations contained in the unaudited condensed consolidated interim financial statements: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Three Months ​ For the Three Months ​ ​ Ended September 30, 2023 ​ Ended September 30, 2022 ​ ​ Income ​ Shares ​ Per Share ​ Income ​ Shares ​ Per Share (in thousands, except per share data) (Numerator) (Denominator) Amount (Numerator) (Denominator) Amount Basic EPS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Earnings available to common shareholders ​ $ 13,536 ​ 45,622 ​ $ 0.30 ​ $ 439,893 ​ 45,379 ​ $ 9.69 Effect of Dilutive Securities ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Stock options and restricted stock units ​ ​ — ​ 443 ​ ​ ​ ​ ​ — ​ 396 ​ ​ ​ Diluted EPS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Earnings available to common shareholders ​ $ 13,536 ​ 46,065 ​ $ 0.29 ​ $ 439,893 ​ 45,775 ​ $ 9.61 Anti-dilutive securities excluded from diluted EPS ​ ​ ​ ​ ​ 145 ​ ​ ​ ​ ​ ​ ​ ​ 435 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Nine Months ​ For the Nine Months ​ ​ Ended September 30, 2023 ​ Ended September 30, 2022 ​ ​ Income ​ Shares ​ Per Share ​ Income ​ Shares ​ Per Share (in thousands, except per share data) (Numerator) (Denominator) Amount (Numerator) (Denominator) Amount Basic EPS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Earnings available to common shareholders ​ $ 189,999 ​ 45,581 ​ $ 4.17 ​ $ 485,577 ​ 45,347 ​ $ 10.71 Effect of Dilutive Securities ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Stock options and restricted stock units ​ ​ — ​ 486 ​ ​ ​ ​ ​ — ​ 428 ​ ​ ​ Diluted EPS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Earnings available to common shareholders ​ $ 189,999 ​ 46,067 ​ $ 4.12 ​ $ 485,577 ​ 45,775 ​ $ 10.61 Anti-dilutive securities excluded from diluted EPS ​ ​ ​ ​ ​ 145 ​ ​ ​ ​ ​ ​ ​ ​ 459 ​ ​ ​ ​
Comprehensive Earnings	G. COMPREHENSIVE EARNINGS ​ Our comprehensive earnings include net earnings plus after-tax unrealized gains and losses on our available-for-sale fixed income portfolio. In reporting the components of comprehensive earnings, we used the federal statutory tax rate of 21 percent. Other comprehensive earnings (loss), as shown in the consolidated statements of earnings and comprehensive earnings, is net of tax benefit of $15 million for the third quarter of 2023, compared to $22 million for the same period in 2022. For the nine-month period ended September 30, 2023, other comprehensive earnings (loss) is net of tax benefit of $10 million, compared to $78 million for the same period in 2022. ​ Unrealized losses, net of tax, recognized in other comprehensive earnings (loss) were $39 million for the first nine months of 2023, compared to $294 million during the same period last year. The unrealized losses in each period were attributable to an increase in interest rates, which decreased the fair value of securities held in the fixed income portfolio. ​ The following table illustrates the changes in the balance of each component of accumulated other comprehensive earnings (loss) for each period presented in the unaudited condensed consolidated interim financial statements: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (in thousands) ​ For the Three Months ​ For the Nine Months ​ ​ Ended September 30, ​ Ended September 30, Unrealized Gains/Losses on Available-for-Sale Securities 2023 2022 2023 2022 Beginning balance ​ $ (211,090) ​ $ (163,318) ​ $ (229,076) ​ $ 49,826 Other comprehensive earnings (loss) before reclassifications ​ ​ (57,261) ​ ​ (82,785) ​ ​ (41,355) ​ ​ (296,422) Amounts reclassified from accumulated other comprehensive earnings ​ ​ 427 ​ ​ 1,537 ​ ​ 2,507 ​ ​ 2,030 Net current-period other comprehensive earnings (loss) ​ $ (56,834) ​ $ (81,248) ​ $ (38,848) ​ $ (294,392) Ending balance ​ $ (267,924) ​ $ (244,566) ​ $ (267,924) ​ $ (244,566) Balance of securities for which an allowance for credit losses has been recognized in net earnings ​ ​ ​ ​ ​ ​ ​ $ 2,005 ​ $ 1,520 ​ Credit losses on or the sale of an available-for-sale security results in amounts being reclassified from accumulated other comprehensive earnings (loss) to current period net earnings. The effects of reclassifications out of accumulated other comprehensive earnings (loss) by the respective line items of net earnings are presented in the following table: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amount Reclassified from Accumulated Other ​ ​ ​ (in thousands) ​ Comprehensive Earnings (Loss) ​ ​ ​ ​ ​ For the Three Months ​ For the Nine Months ​ ​ ​ Component of Accumulated ​ Ended September 30, ​ Ended September 30, ​ ​ Affected line item in the Other Comprehensive Earnings (Loss) 2023 2022 2023 2022 ​ Statement of Earnings Unrealized gains and losses on available-for-sale securities ​ $ (271) ​ $ (1,912) ​ $ (2,808) ​ $ (2,684) ​ ​ Net realized gains (losses) ​ ​ ​ (270) ​ ​ (34) ​ ​ (365) ​ ​ 115 ​ ​ Credit gains (losses) presented within net realized gains ​ ​ $ (541) ​ $ (1,946) ​ $ (3,173) ​ $ (2,569) ​ ​ Earnings (loss) before income taxes ​ ​ ​ 114 ​ ​ 409 ​ ​ 666 ​ ​ 539 ​ ​ Income tax (expense) benefit ​ ​ $ (427) ​ $ (1,537) ​ $ (2,507) ​ $ (2,030) ​ ​ Net earnings (loss)
Fair Value Disclosures	H. FAIR VALUE MEASUREMENTS ​ Fair value is defined as the price in the principal market that would be received for an asset to facilitate an orderly transaction between market participants on the measurement date. We determined the fair value of certain financial instruments based on their underlying characteristics and relevant transactions in the marketplace. We maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. ​ The following are the levels of the fair value hierarchy and a brief description of the type of valuation inputs that are used to establish each level. Financial assets are classified based upon the lowest level of significant input that is used to determine fair value. ​ Level 1 ​ Level 2 ​ Level 3 ​ As a part of management’s process to determine fair value, we utilize widely recognized, third-party pricing sources to determine our fair values. We have obtained an understanding of the third-party pricing sources’ valuation methodologies and inputs. The following is a description of the valuation techniques used for financial assets that are measured at fair value, including the general classification of such assets pursuant to the fair value hierarchy. ​ Corporate, Agencies, Government and Municipal Bonds: ​ Mortgage-backed Securities (MBS)/Commercial Mortgage-backed Securities (CMBS) and Asset-backed Securities (ABS): ​ Regulation D Private Placement Securities: observable and unobservable inputs. Observable inputs include public corporate spread matrices classified by sector, rating and average life, as well as investment and non-investment grade matrices created from fixed income indices. Unobservable inputs include a liquidity spread premium calculated based on public corporate spread and private corporate spread matrices. The quantitative detail of the liquidity spread premium is neither provided nor reasonably available to the Company. An increase to the credit spread assumptions would result in a lower fair value. ​ For all of our fixed income securities classified as Level 2, we periodically conduct a review to assess the reasonableness of the fair values provided by our pricing services. Our review consists of a two-pronged approach. First, we compare prices provided by our pricing services to those provided by an additional source. In some cases, we obtain prices from securities brokers and compare them to the prices provided by our pricing services. If discrepancies are found in our comparisons, we compare our prices to actual reported trade data for like securities. No changes to the fair values supplied by our pricing services have occurred as a result of our reviews. Based on these assessments, we have determined that the fair values of our Level 2 fixed income securities provided by our pricing services are reasonable. ​ Equity Securities: ​ Due to the relatively short-term nature of cash, short-term investments, accounts receivable and accounts payable, their carrying amounts are reasonable estimates of fair value. Our investments in private funds, classified as other invested assets, are measured using the investments’ net asset value per share and are not categorized within the fair value hierarchy.
Risks and Uncertainties	I. RISKS AND UNCERTAINTIES ​ Certain risks and uncertainties are inherent in our day-to-day operations. Adverse changes in the economy, including inflation; rising interest rates; volatile equity markets; and ongoing supply chain disruptions, could lower demand for our insurance products, result in increased levels of loss costs that we could not anticipate at the time we priced our coverages, or negatively impact our investment results, all of which could have an adverse effect on the profitability of our operations. ​ Catastrophe Exposures ​ Our catastrophe reinsurance treaty renewed on January 1, 2023. We purchased limits of $700 million in excess of $25 million first-dollar retention for earthquakes in California, $700 million in excess of $50 million first-dollar retention for earthquakes outside of California and $600 million in excess of $50 million first-dollar retention for all other perils, including wind. These amounts are subject to certain co-participations by the Company on losses in excess of the first-dollar retentions. On June 1, 2023, we purchased $150 million of additional catastrophe reinsurance protection on top of the previously described coverage. This increases the limits to $850 million in excess of $25 million first-dollar retention for earthquakes in California, $850 million in excess of $50 million first-dollar retention for earthquakes outside of California and $750 million in excess of $50 million first-dollar retention for all other perils, including wind, all of which are still subject to certain co-participations in excess of the retentions.

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2023
Accounting Policies [Abstract]
Schedule of Goodwill and Intangible Assets	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ December 31, (in thousands) 2023 2022 Goodwill ​ ​ ​ ​ ​ ​ Surety ​ $ 40,816 ​ $ 40,816 Casualty ​ ​ 5,246 ​ ​ 5,246 Total goodwill ​ $ 46,062 ​ $ 46,062 Indefinite-lived intangibles ​ ​ 7,500 ​ ​ 7,500 Total goodwill and intangibles ​ $ 53,562 ​ $ 53,562
Schedule of Reconciliation of Numerator and Denominator of the Basic and Diluted Earnings Per Share Computations	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Three Months ​ For the Three Months ​ ​ Ended September 30, 2023 ​ Ended September 30, 2022 ​ ​ Income ​ Shares ​ Per Share ​ Income ​ Shares ​ Per Share (in thousands, except per share data) (Numerator) (Denominator) Amount (Numerator) (Denominator) Amount Basic EPS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Earnings available to common shareholders ​ $ 13,536 ​ 45,622 ​ $ 0.30 ​ $ 439,893 ​ 45,379 ​ $ 9.69 Effect of Dilutive Securities ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Stock options and restricted stock units ​ ​ — ​ 443 ​ ​ ​ ​ ​ — ​ 396 ​ ​ ​ Diluted EPS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Earnings available to common shareholders ​ $ 13,536 ​ 46,065 ​ $ 0.29 ​ $ 439,893 ​ 45,775 ​ $ 9.61 Anti-dilutive securities excluded from diluted EPS ​ ​ ​ ​ ​ 145 ​ ​ ​ ​ ​ ​ ​ ​ 435 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Nine Months ​ For the Nine Months ​ ​ Ended September 30, 2023 ​ Ended September 30, 2022 ​ ​ Income ​ Shares ​ Per Share ​ Income ​ Shares ​ Per Share (in thousands, except per share data) (Numerator) (Denominator) Amount (Numerator) (Denominator) Amount Basic EPS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Earnings available to common shareholders ​ $ 189,999 ​ 45,581 ​ $ 4.17 ​ $ 485,577 ​ 45,347 ​ $ 10.71 Effect of Dilutive Securities ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Stock options and restricted stock units ​ ​ — ​ 486 ​ ​ ​ ​ ​ — ​ 428 ​ ​ ​ Diluted EPS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Earnings available to common shareholders ​ $ 189,999 ​ 46,067 ​ $ 4.12 ​ $ 485,577 ​ 45,775 ​ $ 10.61 Anti-dilutive securities excluded from diluted EPS ​ ​ ​ ​ ​ 145 ​ ​ ​ ​ ​ ​ ​ ​ 459 ​ ​ ​ ​
Schedule of Changes in the Balance of Each Component of Accumulated Other Comprehensive Earnings (Loss)	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (in thousands) ​ For the Three Months ​ For the Nine Months ​ ​ Ended September 30, ​ Ended September 30, Unrealized Gains/Losses on Available-for-Sale Securities 2023 2022 2023 2022 Beginning balance ​ $ (211,090) ​ $ (163,318) ​ $ (229,076) ​ $ 49,826 Other comprehensive earnings (loss) before reclassifications ​ ​ (57,261) ​ ​ (82,785) ​ ​ (41,355) ​ ​ (296,422) Amounts reclassified from accumulated other comprehensive earnings ​ ​ 427 ​ ​ 1,537 ​ ​ 2,507 ​ ​ 2,030 Net current-period other comprehensive earnings (loss) ​ $ (56,834) ​ $ (81,248) ​ $ (38,848) ​ $ (294,392) Ending balance ​ $ (267,924) ​ $ (244,566) ​ $ (267,924) ​ $ (244,566) Balance of securities for which an allowance for credit losses has been recognized in net earnings ​ ​ ​ ​ ​ ​ ​ $ 2,005 ​ $ 1,520
Schedule of Effects of Reclassifications out of Accumulated Other Comprehensive Earnings (Loss)	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amount Reclassified from Accumulated Other ​ ​ ​ (in thousands) ​ Comprehensive Earnings (Loss) ​ ​ ​ ​ ​ For the Three Months ​ For the Nine Months ​ ​ ​ Component of Accumulated ​ Ended September 30, ​ Ended September 30, ​ ​ Affected line item in the Other Comprehensive Earnings (Loss) 2023 2022 2023 2022 ​ Statement of Earnings Unrealized gains and losses on available-for-sale securities ​ $ (271) ​ $ (1,912) ​ $ (2,808) ​ $ (2,684) ​ ​ Net realized gains (losses) ​ ​ ​ (270) ​ ​ (34) ​ ​ (365) ​ ​ 115 ​ ​ Credit gains (losses) presented within net realized gains ​ ​ $ (541) ​ $ (1,946) ​ $ (3,173) ​ $ (2,569) ​ ​ Earnings (loss) before income taxes ​ ​ ​ 114 ​ ​ 409 ​ ​ 666 ​ ​ 539 ​ ​ Income tax (expense) benefit ​ ​ $ (427) ​ $ (1,537) ​ $ (2,507) ​ $ (2,030) ​ ​ Net earnings (loss)

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2023
Investments Debt And Equity Securities [Abstract]
Schedule of Disposition of Fixed Income and Equity Securities	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Sales ​ Proceeds ​ Gross Realized ​ Net Realized (in thousands) From Sales Gains Losses Gain (Loss) 2023 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fixed income securities - available-for-sale ​ $ 25,983 ​ $ 145 ​ $ (1,035) ​ $ (890) Equity securities ​ ​ 36,708 ​ ​ 15,990 ​ ​ (101) ​ ​ 15,889 2022 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fixed income securities - available-for-sale ​ $ 48,559 ​ $ 286 ​ $ (2,481) ​ $ (2,195) Equity securities ​ ​ 59,264 ​ ​ 20,410 ​ ​ (609) ​ ​ 19,801 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Calls/Maturities ​ ​ ​ ​ Gross Realized ​ Net Realized (in thousands) Proceeds Gains Losses Gain (Loss) 2023 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fixed income securities - available-for-sale ​ $ 434,263 ​ $ 38 ​ $ (236) ​ $ (198) 2022 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fixed income securities - available-for-sale ​ $ 176,991 ​ $ 142 ​ $ (55) ​ $ 87
Fair Value, Assets Measured on Recurring Basis	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of September 30, 2023 ​ ​ Quoted Prices in ​ Significant Other ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Observable ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Inputs ​ Inputs ​ ​ ​ (in thousands) (Level 1) (Level 2) (Level 3) Total Fixed income securities - available-for-sale ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. government ​ $ — ​ $ 321,611 ​ $ — ​ $ 321,611 U.S. agency ​ ​ — ​ ​ 48,368 ​ ​ — ​ ​ 48,368 Non-U.S. government & agency ​ ​ — ​ ​ 3,811 ​ ​ — ​ ​ 3,811 Agency MBS ​ ​ — ​ ​ 399,381 ​ ​ — ​ ​ 399,381 ABS/CMBS/MBS* ​ ​ — ​ ​ 265,779 ​ ​ — ​ ​ 265,779 Corporate ​ ​ — ​ ​ 1,097,093 ​ ​ 56,291 ​ ​ 1,153,384 Municipal ​ ​ — ​ ​ 525,293 ​ ​ — ​ ​ 525,293 Total fixed income securities - available-for-sale ​ $ — ​ $ 2,661,336 ​ $ 56,291 ​ $ 2,717,627 Equity securities ​ ​ 532,856 ​ ​ — ​ ​ 1,580 ​ ​ 534,436 Total ​ $ 532,856 ​ $ 2,661,336 ​ $ 57,871 ​ $ 3,252,063 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, 2022 ​ ​ Quoted Prices in ​ Significant Other ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Observable ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Inputs ​ Inputs ​ ​ ​ (in thousands) (Level 1) (Level 2) (Level 3) Total Fixed income securities - available-for-sale ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. government ​ $ — ​ $ 454,021 ​ $ — ​ $ 454,021 U.S. agency ​ ​ — ​ ​ 73,063 ​ ​ — ​ ​ 73,063 Non-U.S. government & agency ​ ​ — ​ ​ 5,847 ​ ​ — ​ ​ 5,847 Agency MBS ​ ​ — ​ ​ 331,806 ​ ​ — ​ ​ 331,806 ABS/CMBS/MBS* ​ ​ — ​ ​ 240,736 ​ ​ — ​ ​ 240,736 Corporate ​ ​ — ​ ​ 980,676 ​ ​ 53,654 ​ ​ 1,034,330 Municipal ​ ​ — ​ ​ 527,147 ​ ​ — ​ ​ 527,147 Total fixed income securities - available-for-sale ​ $ — ​ $ 2,613,296 ​ $ 53,654 ​ $ 2,666,950 Equity securities ​ ​ 496,731 ​ ​ 39 ​ ​ 1,612 ​ ​ 498,382 Total ​ $ 496,731 ​ $ 2,613,335 ​ $ 55,266 ​ $ 3,165,332 * Non-agency asset-backed, commercial mortgage-backed and mortgage-backed securities ​
Summary of Changes in Balance of Level 3 Securities	​ ​ ​ ​ ​ (in thousands) Level 3 Securities Balance as of January 1, 2023 ​ $ 55,266 Net realized and unrealized gains (losses) ​ ​ ​ Included in other comprehensive earnings (loss) ​ ​ (377) Purchases ​ ​ 5,475 Sales / Calls / Maturities ​ ​ (2,493) Balance as of September 30, 2023 ​ $ 57,871 Change in unrealized gains (losses) during the period for Level 3 assets held at period-end - included in other comprehensive earnings (loss) ​ $ (377)
Schedule of Contractual Maturity of Securities	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2023 (in thousands) Amortized Cost Fair Value Due in one year or less ​ $ 176,148 ​ $ 174,377 Due after one year through five years ​ ​ 909,286 ​ ​ 858,578 Due after five years through 10 years ​ ​ 653,649 ​ ​ 596,523 Due after 10 years ​ ​ 547,547 ​ ​ 422,989 ABS/CMBS/MBS* ​ ​ 760,155 ​ ​ 665,160 Total available-for-sale ​ $ 3,046,785 ​ $ 2,717,627 * Asset-backed, commercial mortgage-backed and mortgage-backed securities
Schedule of Amortized Cost and Fair Value of Available-for-sale Securities	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2023 ​ ​ Cost or ​ Allowance ​ Gross ​ Gross ​ ​ ​ ​ ​ Amortized ​ for Credit ​ Unrealized ​ Unrealized ​ Fair (in thousands) Cost Losses Gains Losses Value U.S. government ​ $ 334,650 ​ $ — ​ $ — ​ $ (13,039) ​ $ 321,611 U.S. agency ​ ​ 51,477 ​ ​ — ​ ​ — ​ ​ (3,109) ​ ​ 48,368 Non-U.S. government & agency ​ ​ 4,800 ​ ​ — ​ ​ — ​ ​ (989) ​ ​ 3,811 Agency MBS ​ ​ 457,496 ​ ​ — ​ ​ 7 ​ ​ (58,122) ​ ​ 399,381 ABS/CMBS/MBS* ​ ​ 302,659 ​ ​ (4) ​ ​ 65 ​ ​ (36,941) ​ ​ 265,779 Corporate ​ ​ 1,253,152 ​ ​ (700) ​ ​ 952 ​ ​ (100,020) ​ ​ 1,153,384 Municipal ​ ​ 642,551 ​ ​ — ​ ​ 85 ​ ​ (117,343) ​ ​ 525,293 Total Fixed Income ​ $ 3,046,785 ​ $ (704) ​ $ 1,109 ​ $ (329,563) ​ $ 2,717,627 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2022 ​ ​ Cost or ​ Allowance ​ Gross ​ Gross ​ ​ ​ ​ ​ Amortized ​ for Credit ​ Unrealized ​ Unrealized ​ Fair (in thousands) Cost Losses Gains Losses Value U.S. government ​ $ 462,884 ​ $ — ​ $ 8 ​ $ (8,871) ​ $ 454,021 U.S. agency ​ ​ 75,074 ​ ​ — ​ ​ 26 ​ ​ (2,037) ​ ​ 73,063 Non-U.S. government & agency ​ ​ 6,798 ​ ​ — ​ ​ — ​ ​ (951) ​ ​ 5,847 Agency MBS ​ ​ 373,687 ​ ​ — ​ ​ 336 ​ ​ (42,217) ​ ​ 331,806 ABS/CMBS/MBS* ​ ​ 276,126 ​ ​ (8) ​ ​ 62 ​ ​ (35,444) ​ ​ 240,736 Corporate ​ ​ 1,122,097 ​ ​ (331) ​ ​ 541 ​ ​ (87,977) ​ ​ 1,034,330 Municipal ​ ​ 628,607 ​ ​ — ​ ​ 1,265 ​ ​ (102,725) ​ ​ 527,147 Total Fixed Income ​ $ 2,945,273 ​ $ (339) ​ $ 2,238 ​ $ (280,222) ​ $ 2,666,950 * Non-agency asset-backed, commercial mortgage-backed and mortgage-backed securities
Schedule of Debt Securities Available-for-sale Allowance for Credit Loss	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, (in thousands) 2023 2022 2023 2022 Beginning balance ​ $ 434 ​ $ 292 ​ $ 339 ​ $ 441 Increase to allowance from securities for which credit losses were not previously recorded ​ ​ 281 ​ ​ 168 ​ ​ 25 ​ ​ 292 Reduction from securities sold during the period ​ ​ — ​ ​ (166) ​ ​ — ​ ​ (619) Reductions from intent to sell securities ​ ​ — ​ ​ — ​ ​ — ​ ​ (17) Net increase (decrease) from securities that had an allowance at the beginning of the period ​ ​ (11) ​ ​ 33 ​ ​ 340 ​ ​ 230 Balance as of September 30, ​ $ 704 ​ $ 327 ​ $ 704 ​ $ 327
Schedule of Securities in an Unrealized Loss Position Segregated by Type and Length of Time in an Unrealized Loss Position	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2023 ​ December 31, 2022 (in thousands) < 12 Mos. 12 Mos. & Greater Total < 12 Mos. 12 Mos. & Greater Total U.S. government ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 82,683 ​ $ 238,928 ​ $ 321,611 ​ $ 399,361 ​ $ 8,828 ​ $ 408,189 Amortized cost ​ ​ 86,186 ​ ​ 248,464 ​ ​ 334,650 ​ ​ 407,340 ​ ​ 9,720 ​ ​ 417,060 Unrealized loss ​ $ (3,503) ​ $ (9,536) ​ $ (13,039) ​ $ (7,979) ​ $ (892) ​ $ (8,871) U.S. agency ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 15,658 ​ $ 32,710 ​ $ 48,368 ​ $ 32,987 ​ $ 2,170 ​ $ 35,157 Amortized cost ​ ​ 16,304 ​ ​ 35,173 ​ ​ 51,477 ​ ​ 34,627 ​ ​ 2,567 ​ ​ 37,194 Unrealized loss ​ $ (646) ​ $ (2,463) ​ $ (3,109) ​ $ (1,640) ​ $ (397) ​ $ (2,037) Non-U.S. government ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ — ​ $ 3,811 ​ $ 3,811 ​ $ 3,626 ​ $ 2,221 ​ $ 5,847 Amortized cost ​ ​ — ​ ​ 4,800 ​ ​ 4,800 ​ ​ 3,798 ​ ​ 3,000 ​ ​ 6,798 Unrealized Loss ​ $ — ​ $ (989) ​ $ (989) ​ $ (172) ​ $ (779) ​ $ (951) Agency MBS ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 138,173 ​ $ 260,503 ​ $ 398,676 ​ $ 197,252 ​ $ 117,851 ​ $ 315,103 Amortized cost ​ ​ 143,662 ​ ​ 313,136 ​ ​ 456,798 ​ ​ 212,776 ​ ​ 144,544 ​ ​ 357,320 Unrealized loss ​ $ (5,489) ​ $ (52,633) ​ $ (58,122) ​ $ (15,524) ​ $ (26,693) ​ $ (42,217) ABS/CMBS/MBS* ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 41,219 ​ $ 217,289 ​ $ 258,508 ​ $ 96,754 ​ $ 136,149 ​ $ 232,903 Amortized cost ​ ​ 41,882 ​ ​ 253,567 ​ ​ 295,449 ​ ​ 104,724 ​ ​ 163,623 ​ ​ 268,347 Unrealized loss ​ $ (663) ​ $ (36,278) ​ $ (36,941) ​ $ (7,970) ​ $ (27,474) ​ $ (35,444) Corporate ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 274,290 ​ $ 804,638 ​ $ 1,078,928 ​ $ 660,830 ​ $ 323,337 ​ $ 984,167 Amortized cost ​ ​ 287,456 ​ ​ 891,492 ​ ​ 1,178,948 ​ ​ 697,437 ​ ​ 374,707 ​ ​ 1,072,144 Unrealized loss ​ $ (13,166) ​ $ (86,854) ​ $ (100,020) ​ $ (36,607) ​ $ (51,370) ​ $ (87,977) Municipal ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 154,928 ​ $ 358,021 ​ $ 512,949 ​ $ 228,827 ​ $ 204,324 ​ $ 433,151 Amortized cost ​ ​ 159,582 ​ ​ 470,710 ​ ​ 630,292 ​ ​ 255,240 ​ ​ 280,636 ​ ​ 535,876 Unrealized loss ​ $ (4,654) ​ $ (112,689) ​ $ (117,343) ​ $ (26,413) ​ $ (76,312) ​ $ (102,725) Total fixed income ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value ​ $ 706,951 ​ $ 1,915,900 ​ $ 2,622,851 ​ $ 1,619,637 ​ $ 794,880 ​ $ 2,414,517 Amortized cost ​ ​ 735,072 ​ ​ 2,217,342 ​ ​ 2,952,414 ​ ​ 1,715,942 ​ ​ 978,797 ​ ​ 2,694,739 Unrealized loss ​ $ (28,121) ​ $ (301,442) ​ $ (329,563) ​ $ (96,305) ​ $ (183,917) ​ $ (280,222) * Non-agency asset-backed, commercial mortgage-backed and mortgage-backed securities
Schedule of Credit Quality Indicators for Investments in Unrealized Loss Positions	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Equivalent ​ Equivalent ​ (dollars in thousands) ​ ​ ​ NAIC S&P Moody’s ​ Amortized ​ ​ ​ ​ Unrealized ​ Percent ​ Rating Rating Rating Cost Fair Value Loss to Total ​ 1 ​ AAA/AA/A ​ Aaa/Aa/A ​ $ 2,440,094 ​ $ 2,160,142 ​ $ (279,952) ​ 85.0 % 2 ​ BBB ​ Baa ​ ​ 431,695 ​ ​ 388,396 ​ ​ (43,299) ​ 13.1 % 3 ​ BB ​ Ba ​ ​ 48,627 ​ ​ 44,282 ​ ​ (4,345) ​ 1.3 % 4 ​ B ​ B ​ ​ 29,382 ​ ​ 28,045 ​ ​ (1,337) ​ 0.4 % 5 ​ CCC ​ Caa ​ ​ 2,616 ​ ​ 1,986 ​ ​ (630) ​ 0.2 % 6 ​ CC or lower ​ Ca or lower ​ ​ — ​ ​ — ​ ​ — ​ 0.0 % ​ ​ ​ ​ Total ​ $ 2,952,414 ​ $ 2,622,851 ​ $ (329,563) ​ 100.0 %

Historical Loss And LAE Devel_2

Historical Loss And LAE Development (Tables)	9 Months Ended
	Sep. 30, 2023
Historical Loss And L A E Development Disclosure [Abstract]
Schedule of Reconciliation of Unpaid Losses and Settlement Expenses (LAE)	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Nine Months ​ ​ Ended September 30, (in thousands) 2023 2022 Unpaid losses and LAE at beginning of year ​ ​ ​ ​ ​ ​ Gross ​ $ 2,315,637 ​ $ 2,043,555 Ceded ​ ​ (740,089) ​ ​ (608,086) Net ​ $ 1,575,548 ​ $ 1,435,469 ​ ​ ​ ​ ​ ​ ​ Increase (decrease) in incurred losses and LAE ​ ​ ​ ​ ​ ​ Current accident year ​ $ 551,975 ​ $ 491,559 Prior accident years ​ ​ (93,986) ​ ​ (103,032) Total incurred ​ $ 457,989 ​ $ 388,527 ​ ​ ​ ​ ​ ​ ​ Loss and LAE payments for claims incurred ​ ​ ​ ​ ​ ​ Current accident year ​ $ (113,010) ​ $ (52,435) Prior accident years ​ ​ (252,786) ​ ​ (219,682) Total paid ​ $ (365,796) ​ $ (272,117) ​ ​ ​ ​ ​ ​ ​ Net unpaid losses and LAE at September 30, ​ $ 1,667,741 ​ $ 1,551,879 ​ ​ ​ ​ ​ ​ ​ Unpaid losses and LAE at September 30, ​ ​ ​ ​ ​ ​ Gross ​ $ 2,488,695 ​ $ 2,322,737 Ceded ​ ​ (820,954) ​ ​ (770,858) Net ​ $ 1,667,741 ​ $ 1,551,879

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2023
Income Tax Disclosure [Abstract]
Schedule of Reconciliation of Income Tax Expense Attributable to Income from Operations with Amounts Computed by Applying U.S. Federal Tax Rate to Pretax Income from Continuing Operations	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Three Months Ended September 30, 2023 ​ For the Nine Months Ended September 30, 2023 ​ ​ ​ 2023 ​ 2022 ​ 2023 ​ 2022 ​ (in thousands) Amount % Amount % Amount % Amount % ​ Provision for income taxes at the statutory rate of 21% ​ $ 3,154 ​ 21.0 % $ 116,487 ​ 21.0 % $ 49,107 ​ 21.0 % $ 127,401 ​ 21.0 % Increase (reduction) in taxes resulting from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Excess tax benefit on share-based compensation ​ ​ (395) ​ (2.6) % ​ (95) ​ (0.0) % ​ (3,263) ​ (1.4) % ​ (2,270) ​ (0.4) % Tax exempt interest income ​ ​ (270) ​ (1.8) % ​ (280) ​ (0.1) % ​ (827) ​ (0.4) % ​ (850) ​ (0.1) % Dividends received deduction ​ ​ (164) ​ (1.1) % ​ (240) ​ (0.0) % ​ (597) ​ (0.3) % ​ (673) ​ (0.1) % Tax credit ​ ​ (779) ​ (5.2) % ​ (1,461) ​ (0.3) % ​ (2,338) ​ (1.0) % ​ (4,383) ​ (0.7) % ESOP dividends paid deduction ​ ​ (142) ​ (1.0) % ​ (136) ​ (0.0) % ​ (423) ​ (0.2) % ​ (404) ​ (0.0) % Nondeductible expenses ​ ​ 158 ​ 1.1 % ​ 34 ​ 0.0 % ​ 1,505 ​ 0.6 % ​ 608 ​ 0.1 % Other items, net ​ ​ (79) ​ (0.5) % ​ 500 ​ 0.1 % ​ 678 ​ 0.4 % ​ 1,667 ​ 0.2 % Total tax expense ​ $ 1,483 ​ 9.9 % $ 114,809 ​ 20.7 % $ 43,842 ​ 18.7 % $ 121,096 ​ 20.0 %

Stock Based Compensation (Table

Stock Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2023
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Option Activity	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ Aggregate ​ ​ ​ ​ Weighted ​ Average ​ Intrinsic ​ ​ ​ ​ Average ​ Remaining ​ Value ​ Options Exercise Price Contractual Life (in 000’s) Outstanding options at January 1, 2023 ​ 1,695,660 ​ $ 82.42 ​ ​ ​ ​ ​ Options granted ​ 172,250 ​ ​ 135.51 ​ ​ ​ ​ ​ Options exercised ​ (219,905) ​ ​ 57.15 ​ ​ ​ ​ ​ Options canceled/forfeited ​ (8,585) ​ ​ 96.99 ​ ​ ​ ​ ​ Outstanding options at September 30, 2023 ​ 1,639,420 ​ $ 91.31 ​ 4.83 ​ $ 73,168 Exercisable options at September 30, 2023 ​ 849,997 ​ $ 76.60 ​ 3.64 ​ $ 50,400
Summary of Weighted Average Grant-date Assumptions and Weighted Average Fair Values	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2023 2022 ​ Weighted-average fair value of grants ​ $ 26.71 ​ $ 20.99 ​ Risk-free interest rates ​ ​ 3.41 % ​ 2.86 % Dividend yield ​ ​ 2.29 % ​ 2.50 % Expected volatility ​ ​ 22.96 % ​ 22.89 % Expected option life ​ ​ 4.94 years ​ 5.06 years
Schedule of Restricted Stock Units Activity	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ Average ​ ​ ​ ​ Grant Date ​ RSUs Fair Value Nonvested at January 1, 2023 ​ 44,208 ​ $ 109.51 Granted ​ 20,413 ​ ​ 136.19 Reinvested ​ 288 ​ ​ 132.60 Vested ​ (19,957) ​ ​ 103.61 Forfeited ​ (1,795) ​ ​ 116.10 Nonvested at September 30, 2023 ​ 43,157 ​ $ 124.74 ​

Operating Segment Information (

Operating Segment Information (Tables)	9 Months Ended
	Sep. 30, 2023
Segment Reporting [Abstract]
Summary of Selected Information by Operating Segment	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Three Months ​ For the Nine Months Revenues ​ Ended September 30, ​ Ended September 30, (in thousands) 2023 2022 2023 2022 Casualty ​ $ 189,305 ​ $ 179,615 ​ $ 562,384 ​ $ 528,494 Property ​ ​ 94,988 ​ ​ 80,844 ​ ​ 285,596 ​ ​ 222,974 Surety ​ ​ 34,116 ​ ​ 31,009 ​ ​ 100,432 ​ ​ 91,962 Net premiums earned ​ $ 318,409 ​ $ 291,468 ​ $ 948,412 ​ $ 843,430 Net investment income ​ ​ 31,963 ​ ​ 21,270 ​ ​ 87,835 ​ ​ 57,625 Net realized gains ​ ​ 6,558 ​ ​ 573,170 ​ ​ 26,758 ​ ​ 591,562 Net unrealized gains (losses) on equity securities ​ ​ (25,236) ​ ​ (26,414) ​ ​ 15,474 ​ ​ (155,218) Total consolidated revenue ​ $ 331,694 ​ $ 859,494 ​ $ 1,078,479 ​ $ 1,337,399 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net Earnings ​ ​ ​ ​ ​ ​ (in thousands) 2023 2022 2023 2022 Casualty ​ $ 18,670 ​ $ 11,329 ​ $ 57,478 ​ $ 60,418 Property ​ ​ (20,671) ​ ​ (8,308) ​ ​ 33,589 ​ ​ 40,273 Surety ​ ​ 6,248 ​ ​ 5,775 ​ ​ 22,271 ​ ​ 23,539 Net underwriting income (loss) ​ $ 4,247 ​ $ 8,796 ​ $ 113,338 ​ $ 124,230 Net investment income ​ ​ 31,963 ​ ​ 21,270 ​ ​ 87,835 ​ ​ 57,625 Net realized gains ​ ​ 6,558 ​ ​ 573,170 ​ ​ 26,758 ​ ​ 591,562 Net unrealized gains (losses) on equity securities ​ ​ (25,236) ​ ​ (26,414) ​ ​ 15,474 ​ ​ (155,218) General corporate expense and interest on debt ​ ​ (4,245) ​ ​ (4,768) ​ ​ (16,733) ​ ​ (14,587) Equity in earnings (loss) of unconsolidated investees ​ ​ 1,732 ​ ​ (17,352) ​ ​ 7,169 ​ ​ 3,061 Earnings before income taxes ​ $ 15,019 ​ $ 554,702 ​ $ 233,841 ​ $ 606,673 Income tax expense ​ ​ 1,483 ​ ​ 114,809 ​ ​ 43,842 ​ ​ 121,096 Net earnings ​ $ 13,536 ​ $ 439,893 ​ $ 189,999 ​ $ 485,577
Summary of Revenue by Major Product Type	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Three Months ​ For the Nine Months Net Premiums Earned ​ Ended September 30, ​ Ended September 30, (in thousands) 2023 2022 2023 2022 Casualty ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Commercial excess and personal umbrella ​ $ 72,559 ​ $ 65,131 ​ $ 209,402 ​ $ 187,666 General liability ​ ​ 25,250 ​ ​ 25,284 ​ ​ 77,166 ​ ​ 74,130 Commercial transportation ​ ​ 25,843 ​ ​ 24,205 ​ ​ 76,473 ​ ​ 71,774 Professional services ​ ​ 24,882 ​ ​ 24,174 ​ ​ 73,841 ​ ​ 71,590 Small commercial ​ ​ 18,096 ​ ​ 17,005 ​ ​ 54,492 ​ ​ 50,463 Executive products ​ ​ 5,943 ​ ​ 6,556 ​ ​ 18,449 ​ ​ 20,208 Other casualty ​ ​ 16,732 ​ ​ 17,260 ​ ​ 52,561 ​ ​ 52,663 Total ​ $ 189,305 ​ $ 179,615 ​ $ 562,384 ​ $ 528,494 Property ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Commercial property ​ $ 63,783 ​ $ 43,201 ​ $ 173,264 ​ $ 115,419 Marine ​ ​ 33,436 ​ ​ 29,592 ​ ​ 96,484 ​ ​ 84,499 Other property ​ ​ (2,231) ​ ​ 8,051 ​ ​ 15,848 ​ ​ 23,056 Total ​ $ 94,988 ​ $ 80,844 ​ $ 285,596 ​ $ 222,974 Surety ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Commercial ​ $ 12,653 ​ $ 12,057 ​ $ 37,576 ​ $ 35,423 Miscellaneous ​ ​ 11,966 ​ ​ 10,751 ​ ​ 35,900 ​ ​ 33,691 Contract ​ ​ 9,497 ​ ​ 8,201 ​ ​ 26,956 ​ ​ 22,848 Total ​ $ 34,116 ​ $ 31,009 ​ $ 100,432 ​ $ 91,962 Grand Total ​ $ 318,409 ​ $ 291,468 ​ $ 948,412 ​ $ 843,430 ​

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2023
Leases [Abstract]
Schedule of Components of Lease Expense and Other lease Information	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ For the Three Months ​ For the Nine Months ​ ​ Ended September 30, ​ Ended September 30, (in thousands) 2023 2022 2023 2022 Operating lease cost ​ $ 1,235 ​ $ 1,272 ​ $ 3,743 ​ $ 3,686 Variable lease cost ​ ​ 320 ​ ​ 339 ​ ​ 1,202 ​ ​ 1,073 Sublease income ​ ​ (139) ​ ​ (139) ​ ​ (416) ​ ​ (416) Total lease cost ​ $ 1,416 ​ $ 1,472 ​ $ 4,529 ​ $ 4,343 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cash paid for amounts included in measurement of lease liabilities ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Operating cash outflows from operating leases ​ $ 1,431 ​ $ 1,387 ​ $ 4,186 ​ $ 3,987 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ROU assets obtained in exchange for new operating lease liabilities ​ $ 319 ​ $ 54 ​ $ 1,303 ​ $ 2,485 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Reduction to ROU assets resulting from reduction to lease liabilities ​ $ — ​ $ — ​ $ 200 ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Other non-cash reductions to ROU assets ​ $ — ​ $ — ​ $ — ​ $ 73 ​ ​ ​ ​ ​ ​ ​ ​ ​ (in thousands) September 30, 2023 December 31, 2022 ​ Operating lease ROU assets ​ $ 10,339 ​ $ 12,766 ​ Operating lease liabilities ​ $ 11,593 ​ $ 14,499 ​ Weighted-average remaining lease term - operating leases ​ ​ 4.16 years ​ 4.21 years Weighted-average discount rate - operating leases ​ ​ 2.06 % ​ 2.11 % ​
Schedule of Future Minimum Lease Payments under Non-cancellable Leases	​ ​ ​ ​ ​ (in thousands) September 30, 2023 2023 ​ $ 1,205 2024 ​ ​ 3,919 2025 ​ ​ 2,709 2026 ​ ​ 1,744 2027 ​ ​ 997 2028 ​ ​ 704 Thereafter ​ ​ 884 Total future minimum lease payments ​ $ 12,162 Less imputed interest ​ ​ (569) Total operating lease liability ​ $ 11,593 ​

Basis of Presentation and Sum_4

Basis of Presentation and Summary of Significant Accounting Policies - Adopted Accounting Standards (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Sep. 30, 2023
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment in unconsolidated investees	$ 58,275	$ 56,110
Other Investments	47,922	59,730
Retained earnings	1,446,341	$ 1,598,875
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity investments in tax credit structures, unfunded commitment	7,000
Retained earnings	$ (1,000)
Accounting Standards Update [Extensible Enumeration]	rli:AccountingStandardsUpdate202302Member
Cumulative Effect, Period of Adoption, Adjustment | Other Invested Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Investments	$ 11,000

Basis of Presentation and Sum_5

Basis of Presentation and Summary of Significant Accounting Policies - Reinsurance - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Accounting Policies [Abstract]
Amount of allowance for uncollectible amounts on paid recoverables	$ 19,000,000	$ 16,000,000
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	11,131,000	$ 11,250,000
Financing receivable, allowance for credit Loss, write-off	0
Amount of allowance for uncollectible amounts due beyond one year	$ 0
Threshold period for including reinsurance receivables in the allowance for uncollectible amounts	1 year

Basis of Presentation and Sum_6

Basis of Presentation and Summary of Significant Accounting Policies - Goodwill and Intangible Assets - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Business Acquisition
Goodwill	$ 46,062	$ 46,062
Indefinite-lived intangibles	7,500	7,500
Total goodwill and intangibles	53,562	53,562
Surety
Business Acquisition
Goodwill	40,816	40,816
Casualty
Business Acquisition
Goodwill	$ 5,246	$ 5,246

Basis of Presentation and Sum_7

Basis of Presentation and Summary of Significant Accounting Policies - Goodwill and Intangible Assets - Additional Information (Details)	9 Months Ended
	Sep. 30, 2023 USD ($)
Accounting Policies [Abstract]
Goodwill and intangible asset impairment	$ 0

Basis of Presentation and Sum_8

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Reconciliation of Numerator and Denominator of the Basic and Diluted Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Basic EPS, Income (Numerator)
Earnings available to common shareholders	$ 13,536	$ 439,893	$ 189,999	$ 485,577
Diluted EPS, Income (Numerator)
Earnings available to common shareholders	$ 13,536	$ 439,893	$ 189,999	$ 485,577
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	45,622	45,379	45,581	45,347
Effect of Dilutive Securities, Shares (Denominator)
Stock options and restricted stock units	443	396	486	428
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	46,065	45,775	46,067	45,775
Earnings Per Share, Diluted, Other Disclosures
Anti-dilutive securities excluded from diluted EPS	145	435	145	459
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 0.30	$ 9.69	$ 4.17	$ 10.71
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 0.29	$ 9.61	$ 4.12	$ 10.61

Basis of Presentation and Sum_9

Basis of Presentation and Summary of Significant Accounting Policies - Comprehensive Earnings (Loss) - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
COMPREHENSIVE EARNINGS (LOSS)
Tax rate used (as a percent)	21%			21%			21%	21%
Other comprehensive earnings (loss), tax (benefit)	$ (15,000)			$ (22,000)			$ (10,000)	$ (78,000)
Changes in the balance of each component of accumulated other comprehensive earnings (loss)
Other comprehensive earnings (loss), net of tax	$ (56,834)	$ (19,721)	$ 37,707	$ (81,248)	$ (97,563)	$ (115,581)	$ (38,848)	$ (294,392)

Basis of Presentation and Su_10

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Changes in the Balance of Each Component of Accumulated Other Comprehensive Earnings (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Changes in the balance of each component of accumulated other comprehensive earnings (loss)
Beginning balance			$ (229,076)				$ (229,076)
Net current-period other comprehensive earnings (loss)	$ (56,834)	$ (19,721)	37,707	$ (81,248)	$ (97,563)	$ (115,581)	(38,848)	$ (294,392)
Ending balance	(267,924)						(267,924)
Unrealized Gains and Losses on Available-for-Sale Securities
Changes in the balance of each component of accumulated other comprehensive earnings (loss)
Beginning balance	(211,090)		$ (229,076)	(163,318)		$ 49,826	(229,076)	49,826
Other comprehensive earnings (loss) before reclassifications	(57,261)			(82,785)			(41,355)	(296,422)
Amounts reclassified from accumulated other comprehensive earnings	427			1,537			2,507	2,030
Net current-period other comprehensive earnings (loss)	(56,834)			(81,248)			(38,848)	(294,392)
Ending balance	(267,924)	$ (211,090)		(244,566)	$ (163,318)		(267,924)	(244,566)
Balance of securities for which an allowance for credit losses has not been recognized in net earnings	$ 2,005			$ 1,520			$ 2,005	$ 1,520

Basis of Presentation and Su_11

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Effects of Reclassifications out of Accumulated Other Comprehensive Earnings (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss)
Earnings before income taxes	$ 15,019			$ 554,702			$ 233,841	$ 606,673
Income tax (expense) benefit	(1,483)			(114,809)			(43,842)	(121,096)
Net earnings	13,536	$ 77,652	$ 98,811	439,893	$ (2,239)	$ 47,923	189,999	485,577
Unrealized Gains and Losses on Available-for-Sale Securities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss)
Net realized gains (losses) - Available-for-sale	(271)			(1,912)			(2,808)	(2,684)
Credit gains (losses) presented within net realized gains	(270)			(34)			(365)	115
Earnings before income taxes	(541)			(1,946)			(3,173)	(2,569)
Income tax (expense) benefit	114			409			666	539
Net earnings	$ (427)			$ (1,537)			$ (2,507)	$ (2,030)

Basis of Presentation and Su_12

Basis of Presentation and Summary of Significant Accounting Policies - Catastrophe Exposures - Additional Information (Details) - USD ($) $ in Millions	Jun. 01, 2023	Jan. 01, 2023
California Earthquake
Liability For Catastrophe Claims [Line Items]
Catastrophe reinsurance	$ 850	$ 700
First-dollar retention	25	25
Outside of California Earthquake
Liability For Catastrophe Claims [Line Items]
Catastrophe reinsurance	850	700
First-dollar retention	50	50
Other Perils including Wind
Liability For Catastrophe Claims [Line Items]
Catastrophe reinsurance	750	600
First-dollar retention	50	$ 50
Catastrophe
Liability For Catastrophe Claims [Line Items]
Additional catastrophe reinsurance	$ 150

Investments - Disposition of Fi

Investments - Disposition of Fixed Income and Equity Securities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Summary of the disposition of fixed income and equity securities
Fixed income securities - available-for-sale	$ 26,158	$ 46,448
Fixed income securities - available-for-sale	414,529	176,616
Net Realized Gain (Loss) - Fixed income securities - available-for-sale	(2,000)
SALES
Summary of the disposition of fixed income and equity securities
Fixed income securities - available-for-sale	25,983	48,559
Gross Realized Gains - Fixed income securities - available-for-sale	145	286
Gross Realized Losses - Fixed income securities - available-for-sale	(1,035)	(2,481)
Net Realized Gain (Loss) - Fixed income securities - available-for-sale	(890)	(2,195)
Proceeds from Sales - Equity securities	36,708	59,264
Gross Realized Gains - Equity securities	15,990	20,410
Gross Realized Losses - Equity securities	(101)	(609)
Net Realized Gain (Loss) - Equity securities	15,889	19,801
Calls And Maturities Of Marketable Securities
Summary of the disposition of fixed income and equity securities
Fixed income securities - available-for-sale	434,263	176,991
Gross Realized Gains - Fixed income securities - available-for-sale	38	142
Gross Realized Losses - Fixed income securities - available-for-sale	(236)	(55)
Net Realized Gain (Loss) - Fixed income securities - available-for-sale	$ (198)	$ 87

Investments - Assets Measured a

Investments - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Assets measured at Fair Value
Available-for-sale fixed income	$ 2,717,627	$ 2,666,950
Equity securities	534,436	498,382
U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	321,611	454,021
U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	48,368	73,063
Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	3,811	5,847
Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	399,381	331,806
ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	265,779	240,736
Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	1,153,384	1,034,330
Municipal
Assets measured at Fair Value
Available-for-sale fixed income	525,293	527,147
Fair Value Measured on Recurring Basis
Assets measured at Fair Value
Available-for-sale fixed income	2,717,627	2,666,950
Equity securities	534,436	498,382
Total assets at fair value	3,252,063	3,165,332
Fair Value Measured on Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets measured at Fair Value
Equity securities	532,856	496,731
Total assets at fair value	532,856	496,731
Fair Value Measured on Recurring Basis | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	2,661,336	2,613,296
Equity securities		39
Total assets at fair value	2,661,336	2,613,335
Fair Value Measured on Recurring Basis | Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	56,291	53,654
Equity securities	1,580	1,612
Total assets at fair value	57,871	55,266
Fair Value Measured on Recurring Basis | U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	321,611	454,021
Fair Value Measured on Recurring Basis | U.S. government | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	321,611	454,021
Fair Value Measured on Recurring Basis | U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	48,368	73,063
Fair Value Measured on Recurring Basis | U.S. Agency | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	48,368	73,063
Fair Value Measured on Recurring Basis | Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	3,811	5,847
Fair Value Measured on Recurring Basis | Non-U.S. government & agency | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	3,811	5,847
Fair Value Measured on Recurring Basis | Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	399,381	331,806
Fair Value Measured on Recurring Basis | Agency MBS | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	399,381	331,806
Fair Value Measured on Recurring Basis | ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	265,779	240,736
Fair Value Measured on Recurring Basis | ABS/CMBS/MBS | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	265,779	240,736
Fair Value Measured on Recurring Basis | Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	1,153,384	1,034,330
Fair Value Measured on Recurring Basis | Corporate Debt | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	1,097,093	980,676
Fair Value Measured on Recurring Basis | Corporate Debt | Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	56,291	53,654
Fair Value Measured on Recurring Basis | Municipal
Assets measured at Fair Value
Available-for-sale fixed income	525,293	527,147
Fair Value Measured on Recurring Basis | Municipal | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	$ 525,293	$ 527,147

Investments - Summary of Change

Investments - Summary of Changes in Balance of Level 3 securities (Details) - Significant Unobservable Inputs (Level 3) $ in Thousands	9 Months Ended
	Sep. 30, 2023 USD ($)
Summary of changes in Level 3 securities
Beginning balance	$ 55,266
Included in other comprehensive earnings (loss)	(377)
Sales / Calls / Maturities	(2,493)
Purchases	5,475
Ending balance	57,871
Change in unrealized gains (losses) during the period for Level 3 assets held at period-end - OCI	$ (377)

Investments - Amortized Cost an

Investments - Amortized Cost and Fair Value of Available-for-sale Fixed Income Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Amortized Cost
Due in one year or less	$ 176,148
Due after one year through five years	909,286
Due after five years through 10 years	653,649
Due after 10 years	547,547
ABS/CMBS/MBS	760,155
Total available-for-sale	3,046,785	$ 2,945,273
Fair Value
Due in one year or less	174,377
Due after one year through five years	858,578
Due after five years through 10 years	596,523
Due after 10 years	422,989
ABS/CMBS/MBS	665,160
Total available-for-sale	$ 2,717,627	$ 2,666,950

Investments - Additional Inform

Investments - Additional Information (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2023 USD ($) security	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Investment Holdings [Line Items]
Accrued investment income	$ 25,328		$ 21,259
Number of debt securities for which there is an allowance for credit losses | security	19
Losses on fixed income securities	$ 2,000
Number of debt securities in an unrealized loss position for which an allowance for credit losses has not been recorded | security	1,547
Unrealized loss	$ 329,563		280,222
Unrealized losses as percentage of fixed income portfolio cost basis	11%
Unrealized losses relative to total invested assets (as a percent)	10%
Number of unrealized loss positions | security	1,547
Number of securities in unrealized loss positions for 12 months or longer | security	1,200
Maximum
Investment Holdings [Line Items]
Losses on fixed income securities		$ 1,000
Fixed Income Securities
Investment Holdings [Line Items]
Accrued investment income	$ 23,000		$ 20,000

Investments - Amortized Cost _2

Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	$ 3,046,785		$ 2,945,273
Allowance for Credit Losses	(704)	$ (434)	(339)	$ (327)	$ (292)	$ (441)
Gross Unrealized Gains	1,109		2,238
Gross Unrealized Losses	(329,563)		(280,222)
Fair Value	2,717,627		2,666,950
U.S. government
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	334,650		462,884
Gross Unrealized Gains			8
Gross Unrealized Losses	(13,039)		(8,871)
Fair Value	321,611		454,021
U.S. Agency
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	51,477		75,074
Gross Unrealized Gains			26
Gross Unrealized Losses	(3,109)		(2,037)
Fair Value	48,368		73,063
Non-U.S. government & agency
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	4,800		6,798
Gross Unrealized Losses	(989)		(951)
Fair Value	3,811		5,847
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	457,496		373,687
Gross Unrealized Gains	7		336
Gross Unrealized Losses	(58,122)		(42,217)
Fair Value	399,381		331,806
ABS/CMBS/MBS
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	302,659		276,126
Allowance for Credit Losses	(4)		(8)
Gross Unrealized Gains	65		62
Gross Unrealized Losses	(36,941)		(35,444)
Fair Value	265,779		240,736
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	1,253,152		1,122,097
Allowance for Credit Losses	(700)		(331)
Gross Unrealized Gains	952		541
Gross Unrealized Losses	(100,020)		(87,977)
Fair Value	1,153,384		1,034,330
Municipal
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	642,551		628,607
Gross Unrealized Gains	85		1,265
Gross Unrealized Losses	(117,343)		(102,725)
Fair Value	$ 525,293		$ 527,147

Investments - Schedule of Debt

Investments - Schedule of Debt Securities Available-for-sale Allowance for Credit Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Investments Debt And Equity Securities [Abstract]
Beginning balance	$ 434	$ 292	$ 339	$ 441
Increase to allowance from securities for which credit losses were not previously recorded	281	168	25	292
Reduction from securities sold during the period		(166)		(619)
Reductions from intent to sell securities				(17)
Net increase (decrease) from securities that had an allowance at the beginning of the period	(11)	33	340	230
Ending balance	$ 704	$ 327	$ 704	$ 327

Investments - Schedule of Secur

Investments - Schedule of Securities in an Unrealized Loss Position Segregated by Type and Length of Time in an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Fair value
Less than 12 months	$ 706,951	$ 1,619,637
12 months and greater	1,915,900	794,880
Total Fair Value	2,622,851	2,414,517
Amortized Cost
Amortized cost, less than 12 months	735,072	1,715,942
12 months and greater	2,217,342	978,797
Total Cost or Amortized Cost	2,952,414	2,694,739
Unrealized Loss
Less than 12 months	(28,121)	(96,305)
12 months and greater	(301,442)	(183,917)
Total Unrealized Loss	(329,563)	(280,222)
U.S. government
Fair value
Less than 12 months	82,683	399,361
12 months and greater	238,928	8,828
Total Fair Value	321,611	408,189
Amortized Cost
Amortized cost, less than 12 months	86,186	407,340
12 months and greater	248,464	9,720
Total Cost or Amortized Cost	334,650	417,060
Unrealized Loss
Less than 12 months	(3,503)	(7,979)
12 months and greater	(9,536)	(892)
Total Unrealized Loss	(13,039)	(8,871)
U.S. Agency
Fair value
Less than 12 months	15,658	32,987
12 months and greater	32,710	2,170
Total Fair Value	48,368	35,157
Amortized Cost
Amortized cost, less than 12 months	16,304	34,627
12 months and greater	35,173	2,567
Total Cost or Amortized Cost	51,477	37,194
Unrealized Loss
Less than 12 months	(646)	(1,640)
12 months and greater	(2,463)	(397)
Total Unrealized Loss	(3,109)	(2,037)
Non-U.S. government & agency
Fair value
Less than 12 months		3,626
12 months and greater	3,811	2,221
Total Fair Value	3,811	5,847
Amortized Cost
Amortized cost, less than 12 months		3,798
12 months and greater	4,800	3,000
Total Cost or Amortized Cost	4,800	6,798
Unrealized Loss
Less than 12 months		(172)
12 months and greater	(989)	(779)
Total Unrealized Loss	(989)	(951)
Agency MBS
Fair value
Less than 12 months	138,173	197,252
12 months and greater	260,503	117,851
Total Fair Value	398,676	315,103
Amortized Cost
Amortized cost, less than 12 months	143,662	212,776
12 months and greater	313,136	144,544
Total Cost or Amortized Cost	456,798	357,320
Unrealized Loss
Less than 12 months	(5,489)	(15,524)
12 months and greater	(52,633)	(26,693)
Total Unrealized Loss	(58,122)	(42,217)
ABS/CMBS/MBS
Fair value
Less than 12 months	41,219	96,754
12 months and greater	217,289	136,149
Total Fair Value	258,508	232,903
Amortized Cost
Amortized cost, less than 12 months	41,882	104,724
12 months and greater	253,567	163,623
Total Cost or Amortized Cost	295,449	268,347
Unrealized Loss
Less than 12 months	(663)	(7,970)
12 months and greater	(36,278)	(27,474)
Total Unrealized Loss	(36,941)	(35,444)
Corporate Debt
Fair value
Less than 12 months	274,290	660,830
12 months and greater	804,638	323,337
Total Fair Value	1,078,928	984,167
Amortized Cost
Amortized cost, less than 12 months	287,456	697,437
12 months and greater	891,492	374,707
Total Cost or Amortized Cost	1,178,948	1,072,144
Unrealized Loss
Less than 12 months	(13,166)	(36,607)
12 months and greater	(86,854)	(51,370)
Total Unrealized Loss	(100,020)	(87,977)
Municipal
Fair value
Less than 12 months	154,928	228,827
12 months and greater	358,021	204,324
Total Fair Value	512,949	433,151
Amortized Cost
Amortized cost, less than 12 months	159,582	255,240
12 months and greater	470,710	280,636
Total Cost or Amortized Cost	630,292	535,876
Unrealized Loss
Less than 12 months	(4,654)	(26,413)
12 months and greater	(112,689)	(76,312)
Total Unrealized Loss	$ (117,343)	$ (102,725)

Investments - Unrealized Losses

Investments - Unrealized Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Cost or amortized Cost
Total Cost or Amortized Cost	$ 2,952,414	$ 2,694,739
Fair value
Total Fair Value	2,622,851	2,414,517
Unrealized Loss
Total Unrealized Loss	$ (329,563)	$ (280,222)
Percent to Total	100%
NAIC Rating 1 | AAA/AA/A | Aaa/Aa/A
Cost or amortized Cost
Total Cost or Amortized Cost	$ 2,440,094
Fair value
Total Fair Value	2,160,142
Unrealized Loss
Total Unrealized Loss	$ (279,952)
Percent to Total	85%
NAIC Rating 2 | BBB | Baa
Cost or amortized Cost
Total Cost or Amortized Cost	$ 431,695
Fair value
Total Fair Value	388,396
Unrealized Loss
Total Unrealized Loss	$ (43,299)
Percent to Total	13.10%
NAIC Rating 3 | BB | Ba
Cost or amortized Cost
Total Cost or Amortized Cost	$ 48,627
Fair value
Total Fair Value	44,282
Unrealized Loss
Total Unrealized Loss	$ (4,345)
Percent to Total	1.30%
NAIC Rating 4 | B | B
Cost or amortized Cost
Total Cost or Amortized Cost	$ 29,382
Fair value
Total Fair Value	28,045
Unrealized Loss
Total Unrealized Loss	$ (1,337)
Percent to Total	0.40%
NAIC Rating 5 | CCC | Caa
Cost or amortized Cost
Total Cost or Amortized Cost	$ 2,616
Fair value
Total Fair Value	1,986
Unrealized Loss
Total Unrealized Loss	$ (630)
Percent to Total	0.20%
NAIC Rating 6 | CC Or Lower | Ca or lower
Unrealized Loss
Percent to Total	0%

Investments - Debt and Short-te

Investments - Debt and Short-term Investments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Investment Holdings [Line Items]
Other investments	$ 59,730		$ 59,730		$ 47,922
Investment in federal home loan bank stock	us-gaap:AssetPledgedAsCollateralMember		us-gaap:AssetPledgedAsCollateralMember		us-gaap:AssetPledgedAsCollateralMember
Cash and Short-term Investments
Investment in unconsolidated investees	$ 56,110		$ 56,110		$ 58,275
Cash	18,445		18,445		22,818
Short-term investments	125,032		125,032		36,229
Prime Holdings Insurance Services, Inc. (Prime)
Cash and Short-term Investments
Investment in unconsolidated investees	56,000		56,000
Other Unconsolidated Investees | Maximum
Cash and Short-term Investments
Investment in unconsolidated investees	1,000		1,000
FHLBC
Investment Holdings [Line Items]
Investments pledged as collateral	57,000		57,000
Federal home loan bank borrowings fair value disclosure	50,000		50,000
Investment In Low Income Housing Tax Credit Partnerships (LIHTC)
Investment Holdings [Line Items]
Other investments	11,000		11,000		13,000
Amortization on low income housing investments	800	$ 800	2,300	$ 2,500
Total tax benefit on investments in housing tax credit partnership	800	900	2,400	2,600
Qualified affordable housing project investments, unfunded commitment	1,000		1,000
Historic tax credit partnerships (HTC)
Investment Holdings [Line Items]
Other investments	14,000		14,000
Amortization on qualifying equity investments	1,100		3,400
Total tax benefit on investments in housing tax credit partnership	1,400	$ 1,300	4,300	$ 3,900
Equity investments in tax credit structures, unfunded commitment	1,000		1,000
Cash and Short-term Investments
Investment in unconsolidated investees					11,000
Investment in Private Funds
Investment Holdings [Line Items]
Other investments	27,000		27,000		$ 28,000
Fair value investments entities that calculate net asset value per share, unfunded commitments	$ 5,000		$ 5,000

Debt (Details)

Debt (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 15, 2023	Mar. 31, 2023	Sep. 30, 2023	Dec. 31, 2022	Nov. 10, 2021
Debt Instrument [Line Items]
Debt			$ 100,000	$ 199,863
Retirement of senior notes			$ 150,000
FHLBC
Debt Instrument [Line Items]
Amount borrowed from FHLB					$ 50,000
Interest paid monthly at an annualized rate					0.84%
Senior Notes Maturing September 15, 2023
Debt Instrument [Line Items]
Retirement of senior notes	$ 150,000
Line of Credit | PNC
Debt Instrument [Line Items]
Proceeds from line of credit	$ 50,000
Floating interest rate	7.07%
Maximum borrowing capacity - Line of credit		$ 100,000
Maximum borrowing capacity conditional expansion		$ 130,000
Term of facility		3 years
Line of credit facility expiration date		May 29, 2026
Line of Credit | Bank of Montreal, Chicago Branch
Debt Instrument [Line Items]
Maximum borrowing capacity - Line of credit				$ 60,000
Line of credit facility expiration date				Mar. 27, 2023

Historical Loss and LAE Devel_3

Historical Loss and LAE Development - Schedule of Reconciliation of Unpaid Losses and Settlement Expenses (LAE) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Unpaid losses and LAE at beginning of year
Gross	$ 2,315,637	$ 2,043,555
Ceded	(740,089)	(608,086)
Net	1,575,548	1,435,469
Increase (decrease) in incurred losses and LAE
Current accident year	551,975	491,559
Prior accident years	(93,986)	(103,032)
Total incurred	457,989	388,527
Loss and LAE payments for claims incurred
Current accident year	(113,010)	(52,435)
Prior accident years	(252,786)	(219,682)
Total paid	(365,796)	(272,117)
Net unpaid losses and LAE at September 30	1,667,741	1,551,879
Unpaid losses and LAE at September 30
Gross	2,488,695	2,322,737
Ceded	(820,954)	(770,858)
Net unpaid losses and LAE at September 30	$ 1,667,741	$ 1,551,879

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Tax Disclosure [Abstract]
U.S. federal tax rate	21%	21%	21%	21%
Effective income tax rate reconciliation, percent, total	9.90%	20.70%	18.70%	20%

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Income Tax Expense (Benefit) Attributable to Income from Operations with Amounts Computed by Applying U.S. Federal Tax Rate to Pretax Income from Continuing Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Reconciliation of income tax expense reported to amount computed by applying the U.S. federal tax rate
Provision for income taxes at the statutory rate	$ 3,154	$ 116,487	$ 49,107	$ 127,401
Increase (reduction) in taxes resulting from:
Excess tax benefit on share-based compensation	(395)	(95)	(3,263)	(2,270)
Tax exempt interest income	(270)	(280)	(827)	(850)
Dividends received deduction	(164)	(240)	(597)	(673)
Tax credit	(779)	(1,461)	(2,338)	(4,383)
ESOP dividends paid deduction	(142)	(136)	(423)	(404)
Nondeductible expenses	158	34	1,505	608
Other items, net	(79)	500	678	1,667
Total tax expense	$ 1,483	$ 114,809	$ 43,842	$ 121,096
Reconciliation of income tax expense rate to the U.S. federal tax rate
Tax rate used (as a percent)	21%	21%	21%	21%
Excess tax benefit on share-based compensation (as a percent)	(2.60%)	0%	(1.40%)	(0.40%)
Effective rate reduction due to tax exempt interest income (as a percent)	(1.80%)	(0.10%)	(0.40%)	(0.10%)
Effective rate reduction due to dividend received (as a percent)	(1.10%)	(0.00%)	(0.30%)	(0.10%)
Effective rate reduction due to tax credit (as a percent)	(5.20%)	(0.30%)	(1.00%)	(0.70%)
Effective rate reduction due to dividend paid to ESOP (as a percent)	(1.00%)	(0.00%)	(0.20%)	(0.00%)
Effective rate reduction due to nondeductible expenses (as a percent)	1.10%	0%	0.60%	0.10%
Effective rate reduction due to other items, net (as a percent)	(0.50%)	0.10%	0.40%	0.20%
Total tax expense	9.90%	20.70%	18.70%	20%

Stock Based Compensation - Stoc

Stock Based Compensation - Stock Plans - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		5 Months Ended	9 Months Ended		96 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2023	Sep. 30, 2022	May 03, 2023	May 04, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted (in shares)				172,250
Intrinsic value of options				$ 17	$ 6
RLI Corp. Long-Term Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expenses	$ 2	$ 2.4		6.6	6.2
Income tax benefit from stock-based compensation	0.3	$ 0.4		1.1	$ 1
Unrecognized stock-based compensation expense	$ 7		$ 7	$ 7
Stock-based compensation expenses, recognition period				2 years 11 months 19 days
RLI Corp. Long-Term Incentive Plans | Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period				5 years
Term of options				8 years
RLI Corp. Long-Term Incentive Plans | Employee Stock Option | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Age and period of service of the participant to be eligible for retirement				75 years
RLI Corp. Long-Term Incentive Plan (2015 LTIP)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized for grant						4,000,000
Options granted (in shares)						3,291,388
RLI Corp. Long-Term Incentive Plan (2023 LTIP)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized for grant							4,004,891
Options granted (in shares)			165,063

Stock Based Compensation - Summ

Stock Based Compensation - Summary of Option Activity (Details) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2023 USD ($) $ / shares shares	Sep. 30, 2022 USD ($)
Weighted Number of Options Outstanding
Outstanding options, Beginning Balance | shares	1,695,660
Options granted (in shares) | shares	172,250
Options exercised | shares	(219,905)
Options canceled/forfeited | shares	(8,585)
Outstanding options, Ending Balance | shares	1,639,420
Exercisable options, Ending Balance | shares	849,997
Weighted Average Exercise Price
Weighted Average Exercise Price, Outstanding options, Beginning Balance | $ / shares	$ 82.42
Weighted Average Exercise Price, Options granted | $ / shares	135.51
Weighted Average Exercise Price, Options exercised | $ / shares	57.15
Weighted Average Exercise Price, Options canceled/forfeited | $ / shares	96.99
Weighted Average Exercise Price, Outstanding options, Ending Balance | $ / shares	91.31
Weighted Average Exercise Price, Exercisable options | $ / shares	$ 76.60
Weighted Average Remaining Contractual Life
Weighted Average Remaining Contractual Life, Outstanding options	4 years 9 months 29 days
Weighted Average Remaining Contractual Life, Exercisable options	3 years 7 months 20 days
Aggregate Intrinsic Value, Options exercised | $	$ 17,000	$ 6,000
Aggregate Intrinsic Value, Outstanding options | $	73,168
Aggregate Intrinsic Value, Exercisable options | $	$ 50,400

Stock Based Compensation - Su_2

Stock Based Compensation - Summary of Weighted Average Grant-date Assumptions and Weighted Average Fair Values (Details) - $ / shares	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants	$ 26.71	$ 20.99
Risk-free interest rates	3.41%	2.86%
Dividend yield	2.29%	2.50%
Expected volatility	22.96%	22.89%
Expected option life	4 years 11 months 8 days	5 years 21 days

Stock Based Compensation - Rest

Stock Based Compensation - Restricted Stock Units - Additional Information (Details) - Restricted Stock Units (RSUs) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of restricted stock units vested	$ 3	$ 2
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Age and period of service of the participant to be eligible for retirement	75 years
Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period	3 years
Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period	1 year

Stock Based Compensation - Sche

Stock Based Compensation - Schedule of Restricted Stock Units Activity (Details)	9 Months Ended
	Sep. 30, 2023 $ / shares shares
Number of RSUs
RSUs Nonvested, Beginning Balance | shares	44,208
RSUs Granted | shares	20,413
RSUs Reinvested | shares	288
RSUs Vested | shares	(19,957)
RSUs Forfeited | shares	(1,795)
RSUs Nonvested, Ending Balance | shares	43,157
Weighted Average Grant Date Fair Value
Weighted Average Grant Date Fair Value, Nonvested, Beginning Balance | $ / shares	$ 109.51
Weighted Average Grant Date Fair Value, Granted | $ / shares	136.19
Weighted Average Grant Date Fair Value, Reinvested | $ / shares	132.60
Weighted Average Grant Date Fair Value, Vested | $ / shares	103.61
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	116.10
Weighted Average Grant Date Fair Value, Nonvested, Ending Balance | $ / shares	$ 124.74

Operating Segment Information -

Operating Segment Information - Summary of Selected Information by Operating Segment (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenues
Net premiums earned	$ 318,409			$ 291,468			$ 948,412	$ 843,430
Net investment income	31,963			21,270			87,835	57,625
Net realized gains	6,558			573,170			26,758	591,562
Net unrealized gains (losses) on equity securities	(25,236)			(26,414)			15,474	(155,218)
Consolidated revenue	331,694			859,494			1,078,479	1,337,399
Net Earnings
Net underwriting income	4,247			8,796			113,338	124,230
Net investment income	31,963			21,270			87,835	57,625
Net realized gains	6,558			573,170			26,758	591,562
Net unrealized gains (losses) on equity securities	(25,236)			(26,414)			15,474	(155,218)
General corporate expense and interest on debt	(4,245)			(4,768)			(16,733)	(14,587)
Equity in earnings (loss) of unconsolidated investees	1,732			(17,352)			7,169	3,061
Earnings before income taxes	15,019			554,702			233,841	606,673
Income tax expense	1,483			114,809			43,842	121,096
Net earnings	13,536	$ 77,652	$ 98,811	439,893	$ (2,239)	$ 47,923	189,999	485,577
Casualty Segment
Revenues
Net premiums earned	189,305			179,615			562,384	528,494
Net Earnings
Net underwriting income	18,670			11,329			57,478	60,418
Property Segment
Revenues
Net premiums earned	94,988			80,844			285,596	222,974
Net Earnings
Net underwriting income	(20,671)			(8,308)			33,589	40,273
Surety Segment
Revenues
Net premiums earned	34,116			31,009			100,432	91,962
Net Earnings
Net underwriting income	$ 6,248			$ 5,775			$ 22,271	$ 23,539

Operating Segment Information_2

Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Net premiums earned	$ 318,409	$ 291,468	$ 948,412	$ 843,430
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	189,305	179,615	562,384	528,494
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	72,559	65,131	209,402	187,666
Casualty Segment | General liability
Segment Reporting Information [Line Items]
Net premiums earned	25,250	25,284	77,166	74,130
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	25,843	24,205	76,473	71,774
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	24,882	24,174	73,841	71,590
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	18,096	17,005	54,492	50,463
Casualty Segment | Executive products
Segment Reporting Information [Line Items]
Net premiums earned	5,943	6,556	18,449	20,208
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	16,732	17,260	52,561	52,663
Property Segment
Segment Reporting Information [Line Items]
Net premiums earned	94,988	80,844	285,596	222,974
Property Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	63,783	43,201	173,264	115,419
Property Segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	33,436	29,592	96,484	84,499
Property Segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	(2,231)	8,051	15,848	23,056
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	34,116	31,009	100,432	91,962
Surety Segment | Commercial
Segment Reporting Information [Line Items]
Net premiums earned	12,653	12,057	37,576	35,423
Surety Segment | Miscellaneous
Segment Reporting Information [Line Items]
Net premiums earned	11,966	10,751	35,900	33,691
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 9,497	$ 8,201	$ 26,956	$ 22,848

Leases - Schedule of Components

Leases - Schedule of Components of Lease Expense and Other lease Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Leases [Abstract]
Operating lease cost	$ 1,235	$ 1,272	$ 3,743	$ 3,686
Variable lease cost	320	339	1,202	1,073
Sublease income	(139)	(139)	(416)	(416)
Total lease cost	1,416	1,472	4,529	4,343
Cash paid for amounts included in measurement of lease liabilities
Operating cash outflows from operating leases	1,431	1,387	4,186	3,987
ROU assets obtained in exchange for new operating lease liabilities	319	$ 54	1,303	2,485
Reduction to ROU assets resulting from reduction to lease liabilities			200
Other non-cash reductions to ROU assets				$ 73
Operating lease ROU assets	$ 10,339		$ 10,339		$ 12,766
Operating Lease, Right-of-Use Asset, Statement of Financial Position	Other assets		Other assets		Other assets
Operating lease liabilities	$ 11,593		$ 11,593		$ 14,499
Operating Lease, Liability, Statement of Financial Position	Other liabilities		Other liabilities		Other liabilities
Weighted-average remaining lease term - operating leases	4 years 1 month 28 days		4 years 1 month 28 days		4 years 2 months 15 days
Weighted-average discount rate - operating leases	2.06%		2.06%		2.11%

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Lease Payments under Non-cancellable Leases (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Future minimum lease payments under non-cancellable leases
2023	$ 1,205
2024	3,919
2025	2,709
2026	1,744
2027	997
2028	704
Thereafter	884
Total future minimum lease payments	12,162
Less imputed interest	(569)
Operating lease liabilities	$ 11,593	$ 14,499

Acquisitions and Dispositions (

Acquisitions and Dispositions (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2022	Mar. 31, 2023	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Dispositions
Cash proceeds from sale			$ 14,134	$ 686,666
Maui Jim Inc.
Dispositions
Cash proceeds from sale	$ 687,000
Maui Jim Inc. | Net Realized Gain (Loss)
Dispositions
Recognized gain on sale		$ 14,000			$ 571,000

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Pay vs Performance Disclosure
Net Income (Loss)	$ 13,536	$ 77,652	$ 98,811	$ 439,893	$ (2,239)	$ 47,923	$ 189,999	$ 485,577

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
	Sep. 30, 2023
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false