Income Taxes - Deferred Income Taxes (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Sep. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Income Taxes
Net deferred income tax assets			$ 48.4	$ 103.9
Net deferred income tax liabilities			(1,092.5)	(972.4)
Total net deferred income tax liabilities			(1,044.1)	(868.5)
Deferred income tax assets:
Insurance liabilities			26.3	100.8
Investments, including derivative liabilities			205.8	352.7
Net operating and capital loss carryforwards			68.3	78.7
Tax credit carryforwards			307.5	275.7
Employee benefits			314.5	500.8
Foreign currency translation			20.4	105.0
Other deferred income tax assets			42.7	49.1
Gross deferred income tax assets			985.5	1,462.8
Valuation allowance			(5.2)	(7.3)
Total deferred income tax assets			980.3	1,455.5
Deferred income tax liabilities:
Deferred income tax liabilities: Deferred acquisition costs			(587.0)	(899.7)
Investments, including derivative assets			(343.9)	(460.4)
Net unrealized gains on available-for-sale securities			(597.9)	(536.7)
Deferred income tax liabilities: Real estate			(147.1)	(117.5)
Intangible assets			(235.1)	(256.0)
Repatriation toll charge			(19.8)
Other deferred income tax liabilities			(93.6)	(53.7)
Total deferred income tax liabilities			(2,024.4)	(2,324.0)
Total net deferred income tax liabilities			(1,044.1)	(868.5)
Deferred income tax disclosures
One-time deemed repatriation tax			43.0
Accumulated but undistributed earnings from operations of foreign subsidiaries for which U.S. federal and state deferred income taxes have not been provided			1,331.2	1,088.4
Excess book carrying value over tax basis with respect to the original investment of foreign subsidiaries for which U.S. federal and state deferred income taxes have not been provided			106.2
Current income tax receivables associated with outstanding audit issues
Current income tax (receivables) payables associated with outstanding audit issues			(231.8)	(242.9)
Tax return examinations and court rulings
Impact of court ruling on uncertain tax positions		$ (30.3)
Brazil
Deferred income tax disclosures
Transition period for tax increases per enacted tax legislation	3 years
U.S. Federal
Income Taxes
Net deferred income tax liabilities			(803.9)	(733.3)
Deferred income tax assets:
Tax credit carryforwards			307.5	275.7
State
Income Taxes
Net deferred income tax assets			10.9	67.8
Deferred income tax disclosures-operating loss carryforwards
Net operating loss carryforwards			366.1	587.7
Foreign
Income Taxes
Net deferred income tax assets			37.5	36.1
Net deferred income tax liabilities			(288.6)	(239.1)
Deferred income tax disclosures-operating loss carryforwards
Net operating loss carryforwards			125.6	151.6
Valuation allowances on income tax benefits primarily associated with foreign net operating loss carryforwards			$ 5.2	$ 7.3