Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator:
Net income		$ 199,099	$ 249,357	$ 473,134	$ 424,984
Net (income) loss attributable to noncontrolling interests		(13,932)	(38,302)	(34,840)	(59,131)
Net income available to Arch		185,167	211,055	438,294	365,853
Preferred dividends		(11,349)	(5,485)	(22,567)	(10,969)
Net income (loss) available to Arch common shareholders		$ 173,818	$ 205,570	$ 415,727	$ 354,884
Denominator:
Weighted average common shares outstanding		123,009,234	120,599,060	122,145,469	120,513,620
Series D preferred shares		11,477,430	0	12,116,574	0
Weighted average common shares outstanding — basic		134,486,664	120,599,060	134,262,043	120,513,620
Effect of dilutive common share equivalents:
Nonvested restricted shares		1,482,963	1,295,342	1,556,963	1,374,272
Stock options	[1]	3,275,019	2,471,194	3,321,626	2,537,234
Weighted average common shares and common share equivalents outstanding — diluted		139,244,646	124,365,596	139,140,632	124,425,126
Earnings per common share:
Basic (per share)		$ 1.29	$ 1.70	$ 3.10	$ 2.94
Diluted (per share)		$ 1.25	$ 1.65	$ 2.99	$ 2.85
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per common share (shares)		499,999	575,931	764,076	1,027,784

[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 2017 second quarter and 2016 second quarter, the number of stock options excluded were 499,999 and 575,931, respectively. For the six months ended June 30, 2017 and 2016, the number of stock options excluded were 764,076 and 1,027,784, respectively.