Accumulated Other Comprehensive Income (Components of Accumulated Other Comprehensive Income, Net of Tax) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			$ 5,112.0
Change in accumulated other comprehensive (loss) income before reclassifications	$ 230.3	$ (194.6)
Amounts reclassified from accumulated other comprehensive (loss) income	(5.0)	5.0	(3.8)	$ 7.7
Total other comprehensive income (loss)	225.3	(189.6)	484.1	(338.0)
Cumulative effect of change in accounting principles					$ 7.5
Ending balance	5,730.2		5,730.2
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	103.4	51.7	(155.4)	234.0
Change in accumulated other comprehensive (loss) income before reclassifications			487.9	(345.7)
Amounts reclassified from accumulated other comprehensive (loss) income			(3.8)	7.7
Total other comprehensive income (loss)	225.3	(189.6)	484.1	(338.0)
Cumulative effect of change in accounting principles					(33.9)
Ending balance	328.7	(137.9)	328.7	(137.9)
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(365.4)	(272.3)	(375.6)	(281.5)
Change in accumulated other comprehensive (loss) income before reclassifications	(1.2)	(83.1)	9.0	(73.9)
Amounts reclassified from accumulated other comprehensive (loss) income	0.0	0.0	0.0	0.0
Total other comprehensive income (loss)	(1.2)	(83.1)	9.0	(73.9)
Cumulative effect of change in accounting principles					0.0
Ending balance	(366.6)	(355.4)	(366.6)	(355.4)
Net unrealized gains on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	549.9	372.1	301.0	581.2
Change in accumulated other comprehensive (loss) income before reclassifications	233.1	(110.6)	480.4	(288.5)
Amounts reclassified from accumulated other comprehensive (loss) income	(4.6)	4.7	(3.0)	7.4
Total other comprehensive income (loss)	228.5	(105.9)	477.4	(281.1)
Cumulative effect of change in accounting principles					(33.9)
Ending balance	778.4	266.2	778.4	266.2
Net unrealized gains on derivative transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	18.1	21.1	18.4	0.0
Change in accumulated other comprehensive (loss) income before reclassifications	(2.3)	0.5	(2.0)	21.6
Amounts reclassified from accumulated other comprehensive (loss) income	(0.6)	(0.7)	(1.2)	(0.7)
Total other comprehensive income (loss)	(2.9)	(0.2)	(3.2)	20.9
Cumulative effect of change in accounting principles					0.0
Ending balance	15.2	20.9	15.2	20.9
OTTI
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	14.9	14.4	15.1	17.9
Change in accumulated other comprehensive (loss) income before reclassifications	0.7	(1.4)	0.5	(4.9)
Amounts reclassified from accumulated other comprehensive (loss) income	0.0	0.0	0.0	0.0
Total other comprehensive income (loss)	0.7	(1.4)	0.5	(4.9)
Cumulative effect of change in accounting principles					0.0
Ending balance	15.6	13.0	15.6	13.0
Unamortized net (losses) on Pension Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(114.1)	(83.6)	(114.3)	(83.6)
Change in accumulated other comprehensive (loss) income before reclassifications	0.0	0.0	0.0	0.0
Amounts reclassified from accumulated other comprehensive (loss) income	0.2	1.0	0.4	1.0
Total other comprehensive income (loss)	0.2	1.0	0.4	1.0
Cumulative effect of change in accounting principles					$ 0.0
Ending balance	$ (113.9)	$ (82.6)	$ (113.9)	$ (82.6)