Derivative Financial Instruments - Effect of Hedges on Consolidated Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Gains (losses) recognized for hedging relationships
Net investment income (loss)	$ 1,045.5	$ 944.6	$ 2,073.6	$ 1,928.2
Net realized capital gains (losses)	(41.1)	(27.4)	110.3	104.0
Benefits, claims and settlement expenses	1,812.8	1,601.3	3,187.5	4,460.0
Fair Value Hedges \| Interest rate contracts \| Net investment income
Gains (losses) recognized for hedging relationships
Gain (loss) recognized on hedged item	6.9	0.2	(16.5)	11.2
Gain (loss) recognized on derivatives	(6.9)	(0.2)	16.8	(11.5)
Amortization of hedged item basis adjustments	(0.4)	(0.6)	(1.0)	(1.3)
Amounts related to periodic settlements on derivatives	(2.2)	(1.2)	(4.4)	(2.3)
Total gain (loss) recognized for hedging relationships	(2.6)	(1.8)	(5.1)	(3.9)
Cash Flow Hedges \| Net investment income
Gains (losses) recognized for hedging relationships
Total gain (loss) recognized for hedging relationships	6.3	6.8	12.5	13.6
Cash Flow Hedges \| Net realized capital gains (losses)
Gains (losses) recognized for hedging relationships
Total gain (loss) recognized for hedging relationships	0.8	7.9	1.1	8.8
Cash Flow Hedges \| Benefits, claims and settlement expenses
Gains (losses) recognized for hedging relationships
Total gain (loss) recognized for hedging relationships	(0.1)		(0.1)
Cash Flow Hedges \| Interest rate contracts \| Gain (loss) reclassified from AOCI on derivatives
Gains (losses) recognized for hedging relationships
Net investment income (loss)	4.0	4.7	8.1	9.4
Net realized capital gains (losses)		3.9		2.6
Benefits, claims and settlement expenses	0.1		0.1
Gain (loss) reclassified from AOCI as a result that a forecasted transaction is no longer probable of occurring	0.1		0.4
Cash Flow Hedges \| Foreign exchange contracts \| Gain (loss) reclassified from AOCI on derivatives
Gains (losses) recognized for hedging relationships
Net realized capital gains (losses)	0.7	4.0	0.7	6.2
Cash Flow Hedges \| Foreign exchange contracts \| Net investment income
Gains (losses) recognized for hedging relationships
Amounts related to periodic settlements on derivatives	2.3	2.1	4.4	4.2
Net investment hedging relationships
Gains (losses) recognized for hedging relationships
Amount of gain (loss) recognized in accumulated OCI on derivatives	(1.2)	1.8	1.0	9.3
Net investment hedging relationships \| Foreign exchange contracts
Gains (losses) recognized for hedging relationships
Amount of gain (loss) recognized in accumulated OCI on derivatives	$ (1.2)	$ 1.8	$ 1.0	$ 9.3

Amount of periodic settlements on derivatives recognized in statement of operations.

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Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

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Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

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Amount of gain (loss) on derivative instrument designated and qualifying as fair value hedge included in assessment of hedge effectiveness, recognized in earnings.

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Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

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The amount of net gain (loss) reclassified into earnings in the period when cash flow hedge is discontinued because it is probable that the original forecasted transactions will not occur by the end of the original period or an additional two month time period.

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Amount of realized and unrealized gain (loss) on investment.

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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Amount, before tax and reclassification, of gain (loss) from derivative designated and qualifying as net investment hedge.

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Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred.

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