Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income		$ 256,347	$ 199,099	$ 422,731	$ 473,134
Net (income) loss attributable to noncontrolling interests		(12,701)	(13,932)	(28,662)	(34,840)
Net income available to Arch		243,646	185,167	394,069	438,294
Preferred dividends		(10,403)	(11,349)	(20,840)	(22,567)
Loss on redemption of preferred shares		0	0	(2,710)	0
Net income available to Arch common shareholders		$ 233,243	$ 173,818	$ 370,519	$ 415,727
Denominator:
Weighted average common shares outstanding		404,800,421	369,027,702	399,485,135	366,436,407
Series D preferred shares		0	34,432,290	6,677,373	36,349,722
Weighted average common shares outstanding — basic		404,800,421	403,459,992	406,162,508	402,786,129
Effect of dilutive common share equivalents:
Nonvested restricted shares		1,575,749	4,448,889	1,837,356	4,670,889
Stock options	[1]	6,735,035	9,825,057	7,460,892	9,964,878
Weighted average common shares and common share equivalents outstanding — diluted		413,111,205	417,733,938	415,460,756	417,421,896
Earnings per common share:
Basic (per share)		$ 0.58	$ 0.43	$ 0.91	$ 1.03
Diluted (per share)		$ 0.56	$ 0.42	$ 0.89	$ 1.00
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per common share (shares)		5,350,733	1,499,997	5,372,789	2,292,228

[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 2018 second quarter and 2017 second quarter, the number of stock options excluded were 5,350,733 and 1,499,997, respectively. For the six months ended June 30, 2018 and 2017, the number of stock options excluded were 5,372,789 and 2,292,228, respectively.