Consolidated Statements of Comprehensive Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues
Net premiums earned	$ 11,364.8	$ 9,973.5	$ 32,767.3	$ 29,052.8
Investment income	208.9	230.5	639.8	715.5
Net realized gains (losses) on securities:
Net realized gains (losses) on security sales	53.9	195.9	607.8	771.1
Net holding period gains (losses) on securities	(15.9)	336.7	479.8	98.7
Net impairment losses recognized in earnings	(1.2)	0.0	(3.7)	0.0
Total net realized gains (losses) on securities	36.8	532.6	1,083.9	869.8
Fees and other revenues	174.9	151.8	516.8	434.8
Service revenues	73.8	58.8	202.1	169.4
Total revenues	11,859.2	10,947.2	35,209.9	31,242.3
Expenses
Losses and loss adjustment expenses	9,250.7	6,713.1	24,767.6	18,189.7
Policy acquisition costs	951.5	835.2	2,754.7	2,413.5
Other underwriting expenses	1,384.4	1,330.9	4,306.0	4,179.7
Policyholder credit expense	0.0	29.0	0.0	1,062.4
Investment expenses	6.6	4.7	18.5	14.5
Service expenses	72.8	54.4	190.0	154.7
Interest expense	54.2	56.4	167.0	160.8
Total expenses	11,720.2	9,023.7	32,203.8	26,175.3
Net Income
Income before income taxes	139.0	1,923.5	3,006.1	5,067.0
Provision for income taxes	20.5	392.7	617.5	1,046.7
Net income	118.5	1,530.8	2,388.6	4,020.3
Other Comprehensive Income (Loss)
Total net unrealized gains (losses) on fixed-maturity securities	(129.8)	(24.6)	(578.3)	605.6
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax	0.1	0.2	0.6	0.6
Foreign currency translation adjustment	(0.2)	0.0	(0.7)	0.0
Other comprehensive income (loss)	(129.9)	(24.4)	(578.4)	606.2
Comprehensive income (loss)	(11.4)	1,506.4	1,810.2	4,626.5
Computation of Earnings Per Common Share
Net income	118.5	1,530.8	2,388.6	4,020.3
Less: Preferred share dividends	6.7	6.7	20.1	20.1
Net income available to common shareholders	$ 111.8	$ 1,524.1	$ 2,368.5	$ 4,000.2
Average common shares outstanding - Basic	584.7	585.1	584.7	584.9
Net effect of dilutive stock-based compensation	2.4	2.7	2.6	2.8
Total average equivalent common shares - Diluted	587.1	587.8	587.3	587.7
Basic: Earnings per common share	$ 0.19	$ 2.60	$ 4.05	$ 6.84
Diluted: Earnings per common share	$ 0.19	$ 2.59	$ 4.03	$ 6.81