Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Numerator:
Net income (loss)		$ 1,269	$ 669	$ 2,389	$ 1,386
Net (income) loss attributable to noncontrolling interests		0	2	0	0
Net income available to Arch		1,269	671	2,389	1,386
Preferred dividends		(10)	(10)	(20)	(20)
Net income (loss) available to Arch common shareholders		$ 1,259	$ 661	$ 2,369	$ 1,366
Denominator:
Weighted average common shares outstanding — basic		372.7	368.7	371.8	368.0
Effect of dilutive common share equivalents:
Nonvested restricted shares		1.8	2.3	1.9	2.4
Stock options	[1]	7.1	7.4	7.2	7.4
Weighted average common shares and common share equivalents outstanding — diluted		381.6	378.4	380.9	377.8
Earnings per common share:
Basic (per share)		$ 3.38	$ 1.79	$ 6.37	$ 3.71
Diluted (per share)		$ 3.30	$ 1.75	$ 6.22	$ 3.62
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per common share (shares)		0.2	0.3	0.4	0.5

[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 2024 second quarter and 2023 second quarter, the number of stock options excluded were 0.2 million and 0.3 million, respectively. For the six months ended June 30, 2024 and 2023, the number of stock options excluded were 0.4 million and 0.5 million, respectively.