Schedule of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at Beginning of Period	$ (814.2)	$ 127.5	$ 127.5
Other Comprehensive Income (Loss), before Reclassifications, net of tax	317.2	(185.6)
Recognition and Measurement of Financial Assets and Financial Liabilities, Reclassification from AOCI to Retained Earnings			(17.5)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	5.3	6.8
Total Other Comprehensive Income (Loss)	311.9	(192.4)
Balance at End of Period	(502.3)	(82.4)	(814.2)
Net Unrealized Gain on Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at Beginning of Period	(312.4)	607.8	607.8
Other Comprehensive Income (Loss), before Reclassifications, net of tax	305.4	(234.8)
Recognition and Measurement of Financial Assets and Financial Liabilities, Reclassification from AOCI to Retained Earnings			(17.5)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(6.9)	(0.6)
Total Other Comprehensive Income (Loss)	312.3	(234.2)
Balance at End of Period	(0.1)	356.1	(312.4)
Net Gain on Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at Beginning of Period	250.6	282.3	282.3
Other Comprehensive Income (Loss), before Reclassifications, net of tax	(4.7)	2.9
Recognition and Measurement of Financial Assets and Financial Liabilities, Reclassification from AOCI to Retained Earnings		0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	15.8	11.7
Total Other Comprehensive Income (Loss)	(20.5)	(8.8)
Balance at End of Period	230.1	273.5	250.6
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at Beginning of Period	(305.2)	(254.5)	(254.5)
Other Comprehensive Income (Loss), before Reclassifications, net of tax	17.3	47.5
Recognition and Measurement of Financial Assets and Financial Liabilities, Reclassification from AOCI to Retained Earnings			0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0	0.0
Total Other Comprehensive Income (Loss)	17.3	47.5
Balance at End of Period	(287.9)	(207.0)	(305.2)
Unrecognized Pension and Postretirement Benefit Costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at Beginning of Period	(447.2)	(508.1)	(508.1)
Other Comprehensive Income (Loss), before Reclassifications, net of tax	(0.8)	(1.2)
Recognition and Measurement of Financial Assets and Financial Liabilities, Reclassification from AOCI to Retained Earnings			0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(3.6)	(4.3)
Total Other Comprehensive Income (Loss)	2.8	3.1
Balance at End of Period	$ (444.4)	$ (505.0)	$ (447.2)