Stockholders' Equity - Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	$ 153.2	$ 822.0
Other comprehensive income (loss), tax	(65.8)	(142.9)
Other comprehensive income (loss)	87.4	679.1
Net unrealized gains (losses) on available-for-sale securities including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	(507.7)	1,280.8
Adjustments for assumed changes in amortization patterns, pre-tax	(0.2)	0.2
Adjustments for assumed changes in policyholder liabilities, pre-tax	8.2	(0.1)
Other comprehensive income (loss), pre-tax	(456.2)	1,359.4
Other comprehensive income (loss) before reclassifications, tax	108.3	(264.9)
Reclassification from accumulated other comprehensive income, tax	(9.0)	(16.4)
Adjustments for assumed changes in policyholder liabilities, tax	(1.7)
Other comprehensive income (loss), tax	97.6	(281.3)
Other comprehensive income (loss) before reclassifications, after-tax	(399.4)	1,015.9
Reclassification from accumulated other comprehensive income, after-tax	34.5	62.1
Adjustments for assumed changes in amortization patterns, after-tax	(0.2)	0.2
Adjustments for assumed changes in policyholder liabilities, after-tax	6.5	(0.1)
Other comprehensive income (loss)	(358.6)	1,078.1
Net unrealized gains (losses) on available-for-sale securities including NCI | Net realized capital gains (losses)
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax	43.5	78.5
Net unrealized gains (losses) on derivative instruments including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	23.1	3.7
Reclassification from accumulated other comprehensive income, pre-tax	(0.9)	(3.7)
Adjustments for assumed changes in amortization patterns, pre-tax	(0.1)
Other comprehensive income (loss), pre-tax	22.1
Other comprehensive income (loss) before reclassifications, tax	(4.8)	(0.8)
Reclassification from accumulated other comprehensive income, tax	0.2	0.8
Other comprehensive income (loss), tax	(4.6)
Other comprehensive income (loss) before reclassifications, after-tax	18.3	2.9
Reclassification from accumulated other comprehensive income, after-tax	(0.7)	(2.9)
Adjustments for assumed changes in amortization patterns, after-tax	(0.1)
Other comprehensive income (loss)	17.5
Liability for future policy benefits discount rate remeasurement gain (loss)
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	710.2	(682.4)
Other comprehensive income (loss), tax	(155.5)	141.5
Other comprehensive income (loss)	554.7	(540.9)
Market risk benefit nonperformance risk remeasurement gain(loss)
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	(11.0)	7.7
Other comprehensive income (loss), tax	2.3	(1.7)
Other comprehensive income (loss)	(8.7)	6.0
Foreign currency translation adjustment including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	(116.7)	132.2
Other comprehensive income (loss), tax	(4.3)	(0.1)
Other comprehensive income (loss)	(121.0)	132.1
Unrecognized postretirement benefit obligation including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	4.8	5.1
Reclassification from accumulated other comprehensive income, tax	(1.3)	(1.3)
Other comprehensive income (loss), tax	(1.3)	(1.3)
Reclassification from accumulated other comprehensive income, after-tax	3.5	3.8
Other comprehensive income (loss)	3.5	3.8
Unrecognized postretirement benefit obligation including NCI | Operating expense
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax	$ 4.8	$ 5.1