Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Numerator:
Net income (loss)		$ 723	$ 18	$ 2,109	$ 620
Net (income) loss attributable to noncontrolling interests		0	(1)	0	(3)
Net income available to Arch		723	17	2,109	617
Preferred dividends		(10)	(10)	(30)	(30)
Net income (loss) available to Arch common shareholders		$ 713	$ 7	$ 2,079	$ 587
Denominator:
Weighted average common shares outstanding — basic		369.2	365.2	368.4	369.5
Effect of dilutive common share equivalents:
Nonvested restricted shares		2.6	2.3	2.4	2.2
Stock options	[1]	7.6	6.2	7.5	6.7
Weighted average common shares and common share equivalents outstanding — diluted		379.4	373.7	378.3	378.4
Earnings per common share:
Basic (per share)		$ 1.93	$ 0.02	$ 5.64	$ 1.59
Diluted (per share)		$ 1.88	$ 0.02	$ 5.50	$ 1.55
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per common share (shares)		0.3	0.8	0.5	0.8

[1] Certain stock options were not included in the computation of diluted earnings per share where the exercise price of the stock options exceeded the average market price and would have been anti-dilutive or where, when applying the treasury stock method to in-the-money options, the sum of the proceeds, including unrecognized compensation, exceeded the average market price and would have been anti-dilutive. For the 2023 third quarter and 2022 third quarter, the number of stock options excluded were 0.3 million and 0.8 million, respectively. For the nine months ended September 30, 2023 and 2022, the number of stock options excluded were 0.5 million and 0.8 million, respectively.