Revision of Prior Period Financial Statements - Revised Consolidated Statements of Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Revenues
Net earned premiums									$ 8,765.3	$ 8,572.1	$ 8,277.9
Total revenues	$ 2,652.8	$ 2,547.8	$ 2,509.7	$ 2,482.7	$ 2,574.9	$ 2,637.8	$ 2,542.3	$ 2,432.6	10,193.0	10,187.6	9,597.6
Benefits, losses and expenses
Policyholder benefits									2,359.8	2,201.9	2,275.2
Underwriting, general and administrative expenses										3,246.1	3,048.3
Total benefits, losses and expenses									9,843.1	9,416.3	9,019.5
Total pre-tax income	96.3	8.5	69.9	175.2	158.6	188.2	239.7	184.8	349.9	771.3	578.1
Income tax expense									73.3	168.4	58.7
Net income from continuing operations	68.1	7.3	52.2	149.0	124.0	151.0	187.1	140.8	276.6	602.9	519.4
Net income									276.6	1,361.8	441.7
Net income attributable to stockholders									276.6	1,361.8	440.8
Net income attributable to common stockholders, basic	68.1	7.3	52.2	149.0	120.9	879.8	205.8	150.6	276.6	1,357.1	422.1
Net income attributable to common stockholders, diluted	$ 68.1	$ 7.3	$ 52.2	$ 149.0	$ 120.9	$ 879.8	$ 205.8	$ 150.6	$ 276.6	$ 1,357.1	$ 422.1
Basic earnings per share from continuing operations (in dollars per share)	$ 1.27	$ 0.14	$ 0.96	$ 2.67	$ 2.16	$ 2.56	$ 3.07	$ 2.30	$ 5.09	$ 10.11	$ 8.31
Basic earnings per share attributable to common stockholders (in dollars per share)	1.27	0.14	0.96	2.67	2.11	14.88	3.38	2.54	5.09	22.95	7.02
Diluted earnings per share from continuing operations (in dollars per share)	1.27	0.14	0.95	2.65	2.15	2.54	3.05	2.28	5.05	10.03	8.21
Diluted earnings per share attributable to common stockholders (in dollars per share)	$ 1.27	$ 0.14	$ 0.95	$ 2.65	$ 2.09	$ 14.79	$ 3.36	$ 2.51	$ 5.05	$ 22.66	$ 6.98
As Reported
Revenues
Net earned premiums										$ 8,572.1	$ 8,275.8
Total revenues										10,187.6	9,595.5
Benefits, losses and expenses
Policyholder benefits										2,195.7	2,264.9
Underwriting, general and administrative expenses										3,240.6	3,053.8
Total benefits, losses and expenses										9,404.6	9,014.7
Total pre-tax income										783.0	580.8
Income tax expense										169.5	60.4
Net income from continuing operations										613.5	520.4
Net income										1,372.4	442.7
Net income attributable to stockholders										1,372.4	441.8
Net income attributable to common stockholders, basic										1,367.7	423.1
Net income attributable to common stockholders, diluted										$ 1,367.7	$ 423.1
Basic earnings per share from continuing operations (in dollars per share)										$ 10.29	$ 8.33
Basic earnings per share attributable to common stockholders (in dollars per share)										23.13	7.04
Diluted earnings per share from continuing operations (in dollars per share)										10.20	8.22
Diluted earnings per share attributable to common stockholders (in dollars per share)										$ 22.83	$ 6.99
Adjustment
Revenues
Net earned premiums										$ 0.0	$ 2.1
Total revenues										0.0	2.1
Benefits, losses and expenses
Policyholder benefits										6.2	10.3
Underwriting, general and administrative expenses										5.5	(5.5)
Total benefits, losses and expenses										11.7	4.8
Total pre-tax income										(11.7)	(2.7)
Income tax expense										(1.1)	(1.7)
Net income from continuing operations										(10.6)	(1.0)
Net income										(10.6)	(1.0)
Net income attributable to stockholders										(10.6)	(1.0)
Net income attributable to common stockholders, basic										(10.6)	(1.0)
Net income attributable to common stockholders, diluted										$ (10.6)	$ (1.0)
Basic earnings per share from continuing operations (in dollars per share)										$ (0.18)	$ (0.02)
Basic earnings per share attributable to common stockholders (in dollars per share)										(0.18)	(0.02)
Diluted earnings per share from continuing operations (in dollars per share)										(0.17)	(0.01)
Diluted earnings per share attributable to common stockholders (in dollars per share)										$ (0.17)	$ (0.01)