Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net income (loss)		$ (88,674)	$ 495,498
Amounts attributable to noncontrolling interests		232,791	(46,970)
Net income available to Arch		144,117	448,528
Preferred dividends		(10,403)	(10,403)
Net income available to Arch common shareholders		$ 133,714	$ 438,125
Denominator:
Weighted average common shares outstanding — basic		403,892,161	400,184,404
Effect of dilutive common share equivalents:
Nonvested restricted shares		2,275,473	1,807,488
Stock options	[1]	7,865,936	6,979,137
Weighted average common shares and common share equivalents outstanding — diluted		414,033,570	408,971,029
Earnings per common share:
Basic (per share)		$ 0.33	$ 1.09
Diluted (per share)		$ 0.32	$ 1.07
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per common share (shares)		1,155,088	5,577,724

[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 2020 first quarter and 2019 first quarter, the number of stock options excluded were 1,155,088 and 5,577,724, respectively.