Income Taxes	9 Months Ended
	Sep. 30, 2023
Income Tax Disclosure [Abstract]
Income Taxes	The Company’s income tax provision on income before income taxes, including income (loss) from operating affiliates, resulted in an effective tax rate of 8.8% for the nine months ended September 30, 2023, compared to 3.0% for the nine months ended September 30, 2022. The Company’s effective tax rate, which is based upon the expected annual effective tax rate, may fluctuate from period to period based on the relative mix of income or loss reported by jurisdiction and the varying tax rates in each jurisdiction. The effective tax rate for the nine months ended September 30, 2023 and 2022, included discrete income tax benefits of $6 million and $37 million, which had the effect of decreasing the effective tax rate on net income available to Arch common shareholders by 0.3% and 5.7%, respectively. The discrete tax items in the 2023 and 2022 periods were primarily related to equity compensation windfall benefits and the release of a valuation allowance on certain U.K. deferred tax assets, respectively.The Company had a net deferred tax asset of $496 million at September 30, 2023, compared to a net deferred tax asset of $531 million at December 31, 2022. The change is primarily a result of market value fluctuations in the Company’s investment portfolio. In addition, the Company paid $127 million and $202 million of income taxes for the nine months ended September 30, 2023 and 2022, respectively