Income Taxes - Deferred Income Taxes and Other (Details) - USD ($) $ in Millions	12 Months Ended			24 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Income Taxes
U.S. statutory tax rate (as a percent)	21.00%	35.00%	35.00%
Deferred income tax assets:
Insurance liabilities		$ 26.3
Investments, including derivative liabilities	$ 165.6	205.8		$ 165.6
Net operating and capital loss carryforwards	80.8	68.3		80.8
Tax credit carryforwards	163.3	307.5		163.3
Employee benefits	327.3	314.5		327.3
Foreign currency translation	28.1	20.4		28.1
Other deferred income tax assets	33.8	42.7		33.8
Gross deferred income tax assets	798.9	985.5		798.9
Valuation allowance	(17.0)	(5.2)		(17.0)
Total deferred income tax assets	781.9	980.3		781.9
Deferred income tax liabilities:
Deferred acquisition costs	(615.8)	(587.0)		(615.8)
Investments, including derivative assets	(298.1)	(343.9)		(298.1)
Net unrealized gains on available-for-sale securities	(98.1)	(597.9)		(98.1)
Real estate	(144.9)	(147.1)		(144.9)
Intangible assets	(310.0)	(253.9)		(310.0)
Insurance liabilities	(60.2)			(60.2)
Repatriation toll charge		(19.8)
Other deferred income tax liabilities	(71.9)	(74.8)		(71.9)
Total deferred income tax liabilities	(1,599.0)	(2,024.4)		(1,599.0)
Total net deferred income tax liabilities	(817.1)	(1,044.1)		(817.1)
Net deferred income taxes by jurisdiction
Net deferred income tax assets	141.3	48.4		141.3
Net deferred income tax liabilities	(958.4)	(1,092.5)		(958.4)
Total net deferred income tax liabilities	(817.1)	(1,044.1)		(817.1)
Deferred income tax disclosures
One-time deemed repatriation tax, provisional amount		43.0
Adjustment to one-time deemed repatriation tax	5.9
One-time deemed repatriation tax, final determination amount				48.9
Accumulated but undistributed earnings from operations of foreign subsidiaries for which U.S. federal and state deferred income taxes have not been provided	1,184.0	1,331.2		1,184.0
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			106.2
Current income tax receivables associated with outstanding audit issues
Current income tax (receivables) payables associated with outstanding audit issues	(193.0)	(231.8)		(193.0)
U.S. Federal
Deferred income tax assets:
Tax credit carryforwards	163.3	307.5		163.3
Net deferred income taxes by jurisdiction
Net deferred income tax liabilities	(708.2)	(803.9)		(708.2)
State
Net deferred income taxes by jurisdiction
Net deferred income tax assets	108.2	10.9		108.2
Deferred income tax disclosures-operating loss carryforwards
Net operating loss carryforwards	309.6	366.1		309.6
Foreign
Net deferred income taxes by jurisdiction
Net deferred income tax assets	33.1	37.5		33.1
Net deferred income tax liabilities	(250.2)	(288.6)		(250.2)
Deferred income tax disclosures-operating loss carryforwards
Net operating loss carryforwards	191.3	125.6		191.3
Valuation allowances on income tax benefits primarily associated with foreign net operating loss carryforwards	$ 17.0	$ 5.2		$ 17.0