Unaudited Consolidated Statements of Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenues
Net premiums earned	$ 1,948,422	$ 1,744,444
Net investment income	98,856	145,153
Net realized gains (losses)	142,461	(366,960)
Other underwriting income	6,110	6,852
Equity in net income (loss) of investment funds accounted for using the equity method	71,686	(4,209)
Other income (loss)	(1,741)	32
Total revenues	2,265,794	1,525,312
Expenses
Losses and loss adjustment expenses	1,203,100	1,115,419
Acquisition expenses	304,481	247,283
Other operating expenses	261,033	234,544
Corporate expenses	25,384	20,796
Amortization of intangible assets	14,402	16,631
Interest expense	38,346	32,555
Net foreign exchange (gains) losses	(20,063)	(72,671)
Total expenses	1,826,683	1,594,557
Income (loss) before income taxes and income (loss) from operating affiliates	439,111	(69,245)
Income tax expense	(38,860)	(27,945)
Income (loss) from operating affiliates	75,457	8,516
Net income (loss)	475,708	(88,674)
Net (income) loss attributable to noncontrolling interests	(37,552)	232,791
Net income (loss) available to Arch	438,156	144,117
Preferred dividends	(10,403)	(10,403)
Net income (loss) available to Arch common shareholders	$ 427,753	$ 133,714
Net income per common share and common share equivalent
Basic (per share)	$ 1.07	$ 0.33
Diluted (per share)	$ 1.05	$ 0.32
Weighted average common shares and common share equivalents outstanding
Basic (shares)	400,807,895	403,892,161
Diluted (shares)	409,223,253	414,033,570