Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive Income, Net of Tax (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 4,486.0	$ 4,459.7	$ 4,228.7	$ 5,464.1
Change in accumulated other comprehensive loss before reclassifications	(137.2)	(272.5)	(78.6)	(965.9)
Amounts reclassified from accumulated other comprehensive loss	8.9	9.8	13.8	31.0
Total other comprehensive loss	(128.3)	(262.7)	(64.8)	(934.9)
Ending balance	4,490.6	4,098.9	4,490.6	4,098.9
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(922.7)	(822.2)	(986.2)	(150.0)
Total other comprehensive loss	(128.3)	(262.7)	(64.8)	(934.9)
Ending balance	(1,051.0)	(1,084.9)	(1,051.0)	(1,084.9)
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(360.3)	(367.5)	(394.0)	(326.9)
Change in accumulated other comprehensive loss before reclassifications	(11.3)	(51.4)	22.4	(92.0)
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0	0.0	0.0
Total other comprehensive loss	(11.3)	(51.4)	22.4	(92.0)
Ending balance	(371.6)	(418.9)	(371.6)	(418.9)
Net unrealized gains (losses) on investments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(480.9)	(370.1)	(513.2)	256.6
Change in accumulated other comprehensive loss before reclassifications	(123.5)	(221.5)	(102.3)	(874.3)
Amounts reclassified from accumulated other comprehensive loss	10.6	12.2	21.7	38.3
Total other comprehensive loss	(112.9)	(209.3)	(80.6)	(836.0)
Ending balance	(593.8)	(579.4)	(593.8)	(579.4)
Net unrealized gains on derivative transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	12.0	11.1	9.8	12.4
Change in accumulated other comprehensive loss before reclassifications	(2.0)	0.0	1.4	0.0
Amounts reclassified from accumulated other comprehensive loss	0.8	(0.6)	(0.4)	(1.9)
Total other comprehensive loss	(1.2)	(0.6)	1.0	(1.9)
Ending balance	10.8	10.5	10.8	10.5
Unamortized net losses on Pension Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(93.5)	(95.7)	(88.8)	(92.1)
Change in accumulated other comprehensive loss before reclassifications	(0.4)	0.4	(0.1)	0.4
Amounts reclassified from accumulated other comprehensive loss	(2.5)	(1.8)	(7.5)	(5.4)
Total other comprehensive loss	(2.9)	(1.4)	(7.6)	(5.0)
Ending balance	$ (96.4)	$ (97.1)	$ (96.4)	$ (97.1)