Income Taxes - Deferred Income Taxes and Other (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2020
Deferred income tax assets:
Investments, including derivative liabilities	$ 237.0			$ 588.4
Insurance liabilities	143.2			552.4
Net operating and capital loss carryforwards	71.1			69.2
Tax credit carryforwards	106.0			4.6
Employee benefits	338.9			389.1
Foreign currency translation	14.0			9.7
Other deferred income tax assets	45.2			52.5
Gross deferred income tax assets	955.4			1,665.9
Valuation allowance	(16.7)			(18.4)
Total deferred income tax assets	938.7			1,647.5
Deferred income tax liabilities:
Deferred income tax liabilities: Deferred acquisition costs	(562.9)			(522.6)
Investments, including derivative assets	(443.7)			(990.4)
Net unrealized gains on available-for-sale securities	(1,039.9)			(1,762.1)
Deferred income tax liabilities: Real estate	(147.9)			(158.5)
Intangible assets	(342.3)			(387.5)
Other deferred income tax liabilities	(75.0)			(43.4)
Total deferred income tax liabilities	(2,611.7)			(3,864.5)
Total net deferred income tax liabilities	(1,673.0)			(2,217.0)
Net deferred income taxes by jurisdiction
Net deferred income tax assets	123.6			113.8
Net deferred income tax liabilities	(1,796.6)			(2,330.8)
Total net deferred income tax liabilities	(1,673.0)			(2,217.0)
Deferred income tax disclosures
Provisional amount of deemed repatriation tax			$ 43.0
Adjustments to deferred tax assets and liabilities	11.1
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	1,001.5			997.4
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	(195.3)			(54.6)
U.S. Federal
Deferred income tax assets:
Tax credit carryforwards	106.0			4.6
Net deferred income taxes by jurisdiction
Net deferred income tax liabilities	(1,507.6)			(2,011.8)
State
Net deferred income taxes by jurisdiction
Net deferred income tax assets	94.9			81.1
Deferred income tax disclosures-operating loss carryforwards
Net operating loss carryforwards	190.4			269.5
Foreign
Net deferred income taxes by jurisdiction
Net deferred income tax assets	28.7			32.7
Net deferred income tax liabilities	(289.0)			(319.0)
Deferred income tax disclosures-operating loss carryforwards
Net operating loss carryforwards	186.1			170.3
Valuation allowances on income tax benefits primarily associated with foreign net operating loss carryforwards	$ 16.7			$ 18.4