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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			$ 5,652.8
Change in accumulated other comprehensive income (loss) before reclassifications	$ 91.3	$ 78.2	234.5	$ 566.1
Amounts reclassified from accumulated other comprehensive income (loss)	(2.0)	(1.7)	12.2	(5.5)
Total other comprehensive income	89.3	76.5	246.7	560.6
Ending balance	5,936.4		5,936.4
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	568.9	328.7	411.5	(155.4)
Total other comprehensive income	89.3	76.5	246.7	560.6
Ending balance	658.2	405.2	658.2	405.2
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(379.0)	(366.6)	(358.9)	(375.6)
Change in accumulated other comprehensive income (loss) before reclassifications	35.3	(23.6)	(4.5)	(14.6)
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0	19.7	0.0
Total other comprehensive income	35.3	(23.6)	15.2	(14.6)
Ending balance	(343.7)	(390.2)	(343.7)	(390.2)
Net unrealized gains on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	989.1	778.4	856.5	301.0
Change in accumulated other comprehensive income (loss) before reclassifications	53.8	101.0	189.1	581.4
Amounts reclassified from accumulated other comprehensive income (loss)	0.3	(3.6)	(2.4)	(6.6)
Total other comprehensive income	54.1	97.4	186.7	574.8
Ending balance	1,043.2	875.8	1,043.2	875.8
Net unrealized gains on derivative transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	15.9	15.2	17.1	18.4
Change in accumulated other comprehensive income (loss) before reclassifications	0.0	0.5	0.0	(1.5)
Amounts reclassified from accumulated other comprehensive income (loss)	(0.6)	2.2	(1.8)	1.0
Total other comprehensive income	(0.6)	2.7	(1.8)	(0.5)
Ending balance	15.3	17.9	15.3	17.9
Credit related impairment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	0.0	0.0	0.0	0.0
Change in accumulated other comprehensive income (loss) before reclassifications	1.2	0.0	1.2	0.0
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0	0.0	0.0
Total other comprehensive income	1.2	0.0	1.2	0.0
Ending balance	1.2	0.0	1.2	0.0
Non-credit related impairment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	14.3	15.6	15.5	15.1
Change in accumulated other comprehensive income (loss) before reclassifications	1.5	0.2	0.3	0.7
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0	0.0	0.0
Total other comprehensive income	1.5	0.2	0.3	0.7
Ending balance	15.8	15.8	15.8	15.8
Unamortized net (losses) on Pension Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(71.4)	(113.9)	(118.7)	(114.3)
Change in accumulated other comprehensive income (loss) before reclassifications	(0.5)	0.1	48.4	0.1
Amounts reclassified from accumulated other comprehensive income (loss)	(1.7)	(0.3)	(3.3)	0.1
Total other comprehensive income	(2.2)	(0.2)	45.1	0.2
Ending balance	$ (73.6)	$ (114.1)	$ (73.6)	$ (114.1)