Basis of Presentation and Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update	us-gaap:AccountingStandardsUpdate201613Member
Total shareholders' equity available to Arch	$ 11,991,825	$ 11,497,371	$ 10,757,352
Cumulative effect of an accounting change | Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total shareholders' equity available to Arch		$ 22,500