Stockholders' Equity - Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	$ (11,342.0)	$ (903.8)	$ 1,683.6
Other comprehensive income (loss), tax	2,400.9	128.1	(336.6)
Other comprehensive income (loss)	(8,941.1)	(775.7)	1,347.0
Net unrealized gains (losses) on available-for-sale securities including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	(13,167.3)	(2,960.0)	3,441.4
Adjustments for assumed changes in amortization patterns, pre-tax	926.6	171.4	(179.0)
Adjustments for assumed changes in policyholder liabilities, pre-tax	430.0	1,913.1	(1,478.2)
Other comprehensive income (loss), pre-tax	(11,411.8)	(856.1)	1,731.3
Other comprehensive income (loss) before reclassifications, tax	2,791.3	665.4	(728.7)
Reclassification from accumulated other comprehensive income, tax	(87.6)	(5.1)	12.6
Adjustments for assumed changes in amortization patterns, tax	(194.6)	(36.0)	37.6
Adjustments for assumed changes in policyholder liabilities, tax	(86.2)	(439.1)	323.7
Other comprehensive income (loss), tax	2,422.9	185.2	(354.8)
Other comprehensive income (loss) before reclassifications, after-tax	(10,376.0)	(2,294.6)	2,712.7
Reclassification from accumulated other comprehensive income, after-tax	311.3	14.3	(40.3)
Adjustments for assumed changes in amortization patterns, after-tax	732.0	135.4	(141.4)
Adjustments for assumed changes in policyholder liabilities, after-tax	343.8	1,474.0	(1,154.5)
Other comprehensive income (loss)	(8,988.9)	(670.9)	1,376.5
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	398.9	19.4	(52.9)
Net unrealized gains (losses) on derivative instruments including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	(1.4)	66.7	(28.1)
Reclassification from accumulated other comprehensive income, pre-tax	(28.0)	(25.5)	(27.1)
Adjustments for assumed changes in amortization patterns, pre-tax	7.3	(0.2)	2.7
Adjustments for assumed changes in policyholder liabilities, pre-tax	(2.9)	1.6	7.8
Other comprehensive income (loss), pre-tax	(25.0)	42.6	(44.7)
Other comprehensive income (loss) before reclassifications, tax	0.3	(14.0)	6.5
Reclassification from accumulated other comprehensive income, tax	5.8	5.4	5.1
Adjustments for assumed changes in amortization patterns, tax	(1.6)		(0.5)
Adjustments for assumed changes in policyholder liabilities, tax	0.6	(0.4)	(1.6)
Other comprehensive income (loss), tax	5.1	(9.0)	9.5
Other comprehensive income (loss) before reclassifications, after-tax	(1.1)	52.7	(21.6)
Reclassification from accumulated other comprehensive income, after-tax	(22.2)	(20.1)	(22.0)
Adjustments for assumed changes in amortization patterns, after-tax	5.7	(0.2)	2.2
Adjustments for assumed changes in policyholder liabilities, after-tax	(2.3)	1.2	6.2
Other comprehensive income (loss)	(19.9)	33.6	(35.2)
Foreign currency translation adjustment relating to noncontrolling interest | Net income attributable to noncontrolling interest
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax			8.7
Foreign currency translation adjustment including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax		(267.9)	(11.7)
Other comprehensive income (loss), pre-tax	(27.7)	(248.6)	31.3
Other comprehensive income (loss) before reclassifications, tax		(5.6)	(2.6)
Reclassification from accumulated other comprehensive income, tax			1.9
Other comprehensive income (loss), tax	4.8	(5.6)	(0.7)
Other comprehensive income (loss) before reclassifications, after-tax		(273.5)	(14.3)
Reclassification from accumulated other comprehensive income, after-tax		19.3	44.9
Other comprehensive income (loss)	(22.9)	(254.2)	30.6
Foreign currency translation adjustment including NCI | Net realized capital gains (losses)
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax		19.3	43.0
Unrecognized postretirement benefit obligation including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	85.8	106.5	(91.9)
Other comprehensive income (loss), pre-tax	122.5	158.3	(34.3)
Other comprehensive income (loss) before reclassifications, tax	(22.3)	(28.5)	24.9
Reclassification from accumulated other comprehensive income, tax	(9.6)	(14.0)	(15.5)
Other comprehensive income (loss), tax	(31.9)	(42.5)	9.4
Other comprehensive income (loss) before reclassifications, after-tax	63.5	78.0	(67.0)
Reclassification from accumulated other comprehensive income, after-tax	27.1	37.8	42.1
Other comprehensive income (loss)	90.6	115.8	(24.9)
Unrecognized postretirement benefit obligation including NCI | Operating expense
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax	$ 36.7	$ 51.8	$ 57.6