Unaudited Consolidated Statements of Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues
Net premiums written	$ 1,874,144	$ 1,613,457	$ 5,679,701	$ 4,583,614
Change in unearned premiums	(103,052)	(175,434)	(498,811)	(312,998)
Net premiums earned	1,771,092	1,438,023	5,180,890	4,270,616
Net investment income	128,512	161,488	405,150	473,475
Net realized gains (losses)	280,499	61,355	470,127	322,368
Other underwriting income	5,413	3,326	18,932	18,104
Equity in net income (loss) of investment funds accounted for using the equity method	126,735	17,130	57,407	96,533
Other income (loss)	919	1,338	6,327	3,550
Total revenues	2,313,170	1,682,660	6,138,833	5,184,646
Expenses
Losses and loss adjustment expenses	1,216,273	802,455	3,562,214	2,288,530
Acquisition expenses	247,942	211,120	750,014	619,057
Other operating expenses	215,686	196,512	659,479	596,589
Corporate expenses	17,937	17,061	56,653	53,274
Amortization of intangible assets	16,715	20,003	49,835	60,214
Interest expense	41,343	31,328	105,037	89,673
Net foreign exchange (gains) losses	44,885	(33,124)	11,425	(31,697)
Total expenses	1,800,781	1,245,355	5,194,657	3,675,640
Income (loss) before income taxes	512,389	437,305	944,176	1,509,006
Income tax expense	(23,707)	(38,116)	(77,779)	(128,474)
Net income (loss)	488,682	399,189	866,397	1,380,532
Net (income) loss attributable to noncontrolling interests	(69,643)	(6,736)	(4,420)	(70,597)
Net income (loss) available to Arch	419,039	392,453	861,977	1,309,935
Preferred dividends	(10,403)	(10,403)	(31,209)	(31,209)
Net income (loss) available to Arch common shareholders	$ 408,636	$ 382,050	$ 830,768	$ 1,278,726
Net income per common share and common share equivalent
Basic (per share)	$ 1.01	$ 0.95	$ 2.06	$ 3.19
Diluted (per share)	$ 1.00	$ 0.92	$ 2.02	$ 3.11
Weighted average common shares and common share equivalents outstanding
Basic (shares)	402,850,485	402,564,121	403,081,266	401,419,153
Diluted (shares)	409,194,657	413,180,201	410,314,897	410,807,402