Earnings Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Numerator:
Net income	$ 179,145		$ 256,476
Net (income) loss attributable to noncontrolling interests	3,355	[1]	0	[1]
Net income available to Arch	182,500		256,476
Preferred dividends	(5,484)		(5,484)
Net income available to Arch common shareholders	$ 177,016		$ 250,992
Denominator:
Weighted average common shares outstanding — basic	131,857,910		130,907,902
Effect of dilutive common share equivalents:
Nonvested restricted shares	1,346,401		1,290,161
Stock options	3,358,406	[2]	3,211,225	[2]
Weighted average common shares and common share equivalents outstanding — diluted	136,562,717		135,409,288
Earnings per common share:
Basic (per share)	$ 1.34		$ 1.92
Diluted (per share)	$ 1.30		$ 1.85
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per common share (shares)	759,172		1,016,043

[1] See Note 4.

[2] 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 2014 first quarter and 2013 first quarter, the number of stock options excluded were 759,172 and 1,016,043, respectively.