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, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
AOCI:
Beginning balance	$ 6,097.9	$ 5,498.5	$ 5,871.6	$ 5,106.7
Change in accumulated other comprehensive income (loss) before reclassifications	(311.6)	76.0	(388.6)	245.5
Amounts reclassified from accumulated other comprehensive income (loss)	9.5	11.6	30.7	43.6
Total other comprehensive (loss) income	(302.1)	87.6	(357.9)	289.1
Ending balance	5,991.9	5,758.5	5,991.9	5,758.5
Accumulated Other Comprehensive Loss
AOCI:
Beginning balance	(600.0)	(634.6)	(544.2)	(836.1)
Ending balance	(902.1)	(547.0)	(902.1)	(547.0)
Foreign currency translation adjustment
AOCI:
Beginning balance	(337.5)	(362.5)	(351.5)	(415.2)
Change in accumulated other comprehensive income (loss) before reclassifications	(3.4)	7.8	10.6	60.5
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0	0.0	0.0
Total other comprehensive (loss) income	(3.4)	7.8	10.6	60.5
Ending balance	(340.9)	(354.7)	(340.9)	(354.7)
Net unrealized gains (losses) on investments
AOCI:
Beginning balance	(129.9)	(147.5)	(63.0)	(291.9)
Change in accumulated other comprehensive income (loss) before reclassifications	(296.5)	69.3	(384.3)	183.6
Amounts reclassified from accumulated other comprehensive income (loss)	9.4	12.2	30.3	42.3
Total other comprehensive (loss) income	(287.1)	81.5	(354.0)	225.9
Ending balance	(417.0)	(66.0)	(417.0)	(66.0)
Net unrealized gains (losses) on derivative transactions
AOCI:
Beginning balance	(2.2)	4.2	1.8	2.2
Change in accumulated other comprehensive income (loss) before reclassifications	(11.7)	(1.1)	(14.8)	1.4
Amounts reclassified from accumulated other comprehensive income (loss)	(0.5)	(0.8)	(1.4)	(1.3)
Total other comprehensive (loss) income	(12.2)	(1.9)	(16.2)	0.1
Ending balance	(14.4)	2.3	(14.4)	2.3
Unamortized net losses on Pension Plans
AOCI:
Beginning balance	(130.4)	(128.8)	(131.5)	(131.2)
Change in accumulated other comprehensive income (loss) before reclassifications	0.0	0.0	(0.1)	0.0
Amounts reclassified from accumulated other comprehensive income (loss)	0.6	0.2	1.8	2.6
Total other comprehensive (loss) income	0.6	0.2	1.7	2.6
Ending balance	$ (129.8)	$ (128.6)	$ (129.8)	$ (128.6)

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