Other Comprehensive Income (Loss) (Reclassification) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax									$ (2,197.4)	$ (3,374.0)	$ (2,790.0)
Income taxes (Note 13)	$ (1,941.0)	$ (36.0)	$ (252.5)	$ (172.0)	$ (120.2)	$ (192.7)	$ (196.8)	$ (126.7)	(2,401.5)	(636.4)	(381.6)
Net of tax	$ 1,656.9	$ (555.6)	$ (1,008.0)	$ 110.8	$ (771.8)	$ (778.0)	$ (747.7)	$ (440.1)
Other, net of tax									110.1	195.6	341.5
Total reclassifications for the period, net of tax									60.5	250.7	116.2
Amortization of retirement benefit items: | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of retirement benefit items:									306.6	304.7	421.2
Total before tax									222.3	230.7	340.5
Income taxes (Note 13)									68.9	71.5	105.6
Net of tax									153.4	159.2	234.9
Prior service benefits, net | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of retirement benefit items:									(84.3)	(74.0)	(80.7)
Unrealized Net Gains (Losses) on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period, net of tax									(110.6)	7.2	(128.2)
Unrealized Net Gains (Losses) on Securities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax									170.2	(11.2)	197.3
Income taxes (Note 13)									59.6	(4.0)	69.1
Net of tax									110.6	(7.2)	128.2
Other, net of tax									(170.2)	(16.1)	(209.3)
Impairment losses | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other, net of tax									0.0	27.3	12.0
Other, net of tax | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other, net of tax									17.7	84.3	9.5
Foreign Currency Translation Gains (Losses)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period, net of tax									$ 8.1	74.5	$ 0.0
Foreign Currency Translation Gains (Losses) | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other, net of tax										$ (74.5)