Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues
Net premiums earned	$ 8,082,298	$ 6,991,935	$ 5,786,498
Net investment income	389,118	519,608	627,738
Net realized gains (losses)	379,845	823,460	363,198
Other underwriting income	22,073	26,784	24,861
Equity in net income of investments accounted for using the equity method	366,402	146,693	123,672
Other income (loss)	10,244	29	0
Total revenues	9,249,980	8,508,509	6,925,967
Expenses
Losses and loss adjustment expenses	4,584,803	4,689,599	3,133,452
Acquisition expenses	1,303,178	1,004,842	840,945
Other operating expenses	998,595	875,176	800,997
Corporate expenses	79,157	81,988	80,111
Amortization of intangible assets	82,955	69,031	82,104
Interest expense	139,470	143,456	120,872
Net foreign exchange losses (gains)	(41,529)	83,634	20,609
Total expenses	7,146,629	6,947,726	5,079,090
Income (loss) before income taxes and income (loss) from operating affiliates	2,103,351	1,560,783	1,846,877
Income taxes:
Current tax expense (benefit)	295,533	197,662	144,361
Deferred tax expense (benefit)	(166,951)	(85,824)	11,449
Income tax expense	128,582	111,838	155,810
Income (loss) from operating affiliates	264,693	16,766	2,233
Net income	2,239,462	1,465,711	1,693,300
Net (income) loss attributable to noncontrolling interests	(82,613)	(60,190)	(56,981)
Net income available to Arch	2,156,849	1,405,521	1,636,319
Preferred share dividends	(48,343)	(41,612)	(41,612)
Loss on redemption of preferred shares	(15,101)	0	0
Net income available to Arch common shareholders	$ 2,093,405	$ 1,363,909	$ 1,594,707
Net income per common share and common share equivalent
Basic (per share)	$ 5.34	$ 3.38	$ 3.97
Diluted (per share)	$ 5.23	$ 3.32	$ 3.87
Weighted average common shares and common share equivalents outstanding
Basic (shares)	391,748,715	403,062,179	401,802,815
Diluted (shares)	400,345,936	410,259,455	411,609,478