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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance				$ 4,270.6
Change in accumulated other comprehensive (loss) income before reclassifications		$ (194.6)	$ 95.1	(345.7)	$ 152.8
Amounts reclassified from accumulated other comprehensive (loss) income		5.0	(6.5)	7.7	(10.5)
Total other comprehensive (loss) income		(189.6)	88.6	(338.0)	142.3
Ending balance		5,312.5		5,312.5
Accounting Standards Update 2016-01 and 2014-09
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative effect of change in accounting principles	[1]	7.5		7.5
Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		51.7	148.3	234.0	94.6
Total other comprehensive (loss) income				(338.0)
Ending balance		(137.9)	236.9	(137.9)	236.9
Accumulated Other Comprehensive Income | Accounting Standards Update 2016-01 and 2014-09
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative effect of change in accounting principles	[1]	(33.9)		(33.9)
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(272.3)	(300.3)	(281.5)	(322.1)
Change in accumulated other comprehensive (loss) income before reclassifications		(83.1)	15.7	(73.9)	37.5
Amounts reclassified from accumulated other comprehensive (loss) income		0.0	0.0	0.0	0.0
Total other comprehensive (loss) income		(83.1)	15.7	(73.9)	37.5
Ending balance		(355.4)	(284.6)	(355.4)	(284.6)
Foreign currency translation adjustment | Accounting Standards Update 2016-01 and 2014-09
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative effect of change in accounting principles		0.0		0.0
Net unrealized gains on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		372.1	491.7	581.2	459.3
Change in accumulated other comprehensive (loss) income before reclassifications		(110.6)	80.9	(288.5)	117.1
Amounts reclassified from accumulated other comprehensive (loss) income		4.7	(7.0)	7.4	(10.8)
Total other comprehensive (loss) income		(105.9)	73.9	(281.1)	106.3
Ending balance		266.2	565.6	266.2	565.6
Net unrealized gains on securities | Accounting Standards Update 2016-01 and 2014-09
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative effect of change in accounting principles		(33.9)		(33.9)
Net unrealized gains on derivative transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		21.1	0.0	0.0	0.0
Change in accumulated other comprehensive (loss) income before reclassifications		0.5	0.0	21.6	0.0
Amounts reclassified from accumulated other comprehensive (loss) income		(0.7)	0.0	(0.7)	0.0
Total other comprehensive (loss) income		(0.2)	0.0	20.9	0.0
Ending balance		20.9	0.0	20.9	0.0
Net unrealized gains on derivative transactions | Accounting Standards Update 2016-01 and 2014-09
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative effect of change in accounting principles		0.0		0.0
OTTI
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		14.4	20.3	17.9	20.6
Change in accumulated other comprehensive (loss) income before reclassifications		(1.4)	(1.5)	(4.9)	(1.8)
Amounts reclassified from accumulated other comprehensive (loss) income		0.0	0.0	0.0	0.0
Total other comprehensive (loss) income		(1.4)	(1.5)	(4.9)	(1.8)
Ending balance		13.0	18.8	13.0	18.8
OTTI | Accounting Standards Update 2016-01 and 2014-09
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative effect of change in accounting principles		0.0		0.0
Unamortized net (losses) on Pension Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(83.6)	(63.4)	(83.6)	(63.2)
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		1.0	0.5	1.0	0.3
Total other comprehensive (loss) income		1.0	0.5	1.0	0.3
Ending balance		(82.6)	$ (62.9)	(82.6)	$ (62.9)
Unamortized net (losses) on Pension Plans | Accounting Standards Update 2016-01 and 2014-09
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative effect of change in accounting principles		$ 0.0		$ 0.0

[1] Amounts relate to 1) the requirement to recognize the fair value changes of equity securities directly within income (resulting in a reclassification of unrealized gains as of December 31, 2017 between accumulated other comprehensive income ("AOCI") and retained earnings) and 2) the impact of adoption of the new revenue recognition standard for revenues from service contracts and sales of products. See Note 3 for additional information.