Stockholders' Equity - Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	$ (715.7)	$ (1,598.9)	$ 70.0	$ (6,824.4)
Other comprehensive income (loss), tax	117.9	324.6	(32.5)	1,411.2
Other comprehensive income (loss)	(597.8)	(1,274.3)	37.5	(5,413.2)
Net unrealized gains (losses) on available-for-sale securities including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	(2,333.5)	(3,653.1)	(1,840.4)	(14,038.6)
Adjustments for assumed changes in amortization patterns, pre-tax	(1.8)	(1.1)	(2.5)	(3.8)
Adjustments for assumed changes in policyholder liabilities, pre-tax	0.6	0.3	0.7	273.1
Other comprehensive income (loss), pre-tax	(2,304.4)	(3,577.6)	(1,628.2)	(13,685.5)
Other comprehensive income (loss) before reclassifications, tax	501.0	778.2	398.8	2,975.2
Reclassification from accumulated other comprehensive income, tax	(6.2)	(15.6)	(44.7)	(21.0)
Adjustments for assumed changes in amortization patterns, tax	0.4	0.2	0.6	0.8
Adjustments for assumed changes in policyholder liabilities, tax	(0.2)	(0.1)	(0.2)	(57.5)
Other comprehensive income (loss), tax	495.0	762.7	354.5	2,897.5
Other comprehensive income (loss) before reclassifications, after-tax	(1,832.5)	(2,874.9)	(1,441.6)	(11,063.4)
Reclassification from accumulated other comprehensive income, after-tax	24.1	60.7	169.3	62.8
Adjustments for assumed changes in amortization patterns, after-tax	(1.4)	(0.9)	(1.9)	(3.0)
Adjustments for assumed changes in policyholder liabilities, after-tax	0.4	0.2	0.5	215.6
Other comprehensive income (loss)	(1,809.4)	(2,814.9)	(1,273.7)	(10,788.0)
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	30.3	76.3	214.0	83.8
Net unrealized gains (losses) on derivative instruments including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	(16.8)	104.6	(19.3)	84.2
Reclassification from accumulated other comprehensive income, pre-tax	(1.3)	(4.4)	(6.2)	(21.8)
Adjustments for assumed changes in amortization patterns, pre-tax	(0.9)		(0.9)	(0.1)
Adjustments for assumed changes in policyholder liabilities, pre-tax	0.2		0.2	0.4
Other comprehensive income (loss), pre-tax	(18.8)	100.2	(26.2)	62.7
Other comprehensive income (loss) before reclassifications, tax	3.5	(22.0)	4.1	(17.7)
Reclassification from accumulated other comprehensive income, tax	0.3	1.0	1.3	4.6
Adjustments for assumed changes in amortization patterns, tax	0.2		0.2
Adjustments for assumed changes in policyholder liabilities, tax	(0.1)		(0.1)	(0.1)
Other comprehensive income (loss), tax	3.9	(21.0)	5.5	(13.2)
Other comprehensive income (loss) before reclassifications, after-tax	(13.3)	82.6	(15.2)	66.5
Reclassification from accumulated other comprehensive income, after-tax	(1.0)	(3.4)	(4.9)	(17.2)
Adjustments for assumed changes in amortization patterns, after-tax	(0.7)		(0.7)	(0.1)
Adjustments for assumed changes in policyholder liabilities, after-tax	0.1		0.1	0.3
Other comprehensive income (loss)	(14.9)	79.2	(20.7)	49.5
Liability for future policy benefits discount rate remeasurement gain (loss)
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	1,773.5	1,936.7	1,746.1	6,775.1
Other comprehensive income (loss), tax	(399.9)	(425.1)	(404.2)	(1,435.1)
Other comprehensive income (loss)	1,373.6	1,511.6	1,341.9	5,340.0
Market risk benefit nonperformance risk gain (loss)
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	(22.6)	(3.7)	(22.2)	165.7
Other comprehensive income (loss), tax	4.7	0.8	4.7	(34.7)
Other comprehensive income (loss)	(17.9)	(2.9)	(17.5)	131.0
Foreign currency translation adjustment including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	(149.1)		1.6
Reclassification from accumulated other comprehensive income, pre-tax	0.1		0.4
Other comprehensive income (loss), pre-tax	(149.0)	(63.8)	2.0	(172.2)
Other comprehensive income (loss) before reclassifications, tax	15.6		6.6
Other comprehensive income (loss), tax	15.6	9.5	6.6	4.5
Other comprehensive income (loss) before reclassifications, after-tax	(133.5)		8.2
Reclassification from accumulated other comprehensive income, after-tax	0.1		0.4
Other comprehensive income (loss)	(133.4)	(54.3)	8.6	(167.7)
Unrecognized postretirement benefit obligation including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax		0.1	(18.3)	2.2
Other comprehensive income (loss), pre-tax	5.6	9.3	(1.5)	29.8
Other comprehensive income (loss) before reclassifications, tax	0.1	(0.1)	4.8	(0.6)
Reclassification from accumulated other comprehensive income, tax	(1.5)	(2.2)	(4.4)	(7.2)
Other comprehensive income (loss), tax	(1.4)	(2.3)	0.4	(7.8)
Other comprehensive income (loss) before reclassifications, after-tax	0.1		(13.5)	1.6
Reclassification from accumulated other comprehensive income, after-tax	4.1	7.0	12.4	20.4
Other comprehensive income (loss)	4.2	7.0	(1.1)	22.0
Unrecognized postretirement benefit obligation including NCI | Operating expense
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax	$ 5.6	$ 9.2	$ 16.8	$ 27.6