Stockholders' Equity - Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	$ (4,519.6)	$ (1,799.1)
Other comprehensive income (loss), tax	989.5	369.6
Other comprehensive income (loss)	(3,530.1)	(1,429.5)
Net unrealized gains (losses) on available-for-sale securities including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	(5,542.9)	(3,276.7)
Adjustments for assumed changes in amortization patterns, pre-tax	344.9	144.6
Adjustments for assumed changes in policyholder liabilities, pre-tax	575.6	1,415.3
Other comprehensive income (loss), pre-tax	(4,663.0)	(1,744.2)
Other comprehensive income (loss) before reclassifications, tax	1,175.1	700.5
Reclassification from accumulated other comprehensive income, tax	5.2	5.7
Adjustments for assumed changes in amortization patterns, tax	(72.4)	(30.4)
Adjustments for assumed changes in policyholder liabilities, tax	(118.8)	(305.0)
Other comprehensive income (loss), tax	989.1	370.8
Other comprehensive income (loss) before reclassifications, after-tax	(4,367.8)	(2,576.2)
Reclassification from accumulated other comprehensive income, after-tax	(35.4)	(21.7)
Adjustments for assumed changes in amortization patterns, after-tax	272.5	114.2
Adjustments for assumed changes in policyholder liabilities, after-tax	456.8	1,110.3
Other comprehensive income (loss)	(3,673.9)	(1,373.4)
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	(40.6)	(27.4)
Net unrealized gains (losses) on derivative instruments including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	(21.0)	0.7
Reclassification from accumulated other comprehensive income, pre-tax	(6.8)	(4.4)
Adjustments for assumed changes in amortization patterns, pre-tax	2.3
Adjustments for assumed changes in policyholder liabilities, pre-tax	5.0	1.0
Other comprehensive income (loss), pre-tax	(20.5)	(2.7)
Other comprehensive income (loss) before reclassifications, tax	4.4	(0.1)
Reclassification from accumulated other comprehensive income, tax	1.4	0.9
Adjustments for assumed changes in amortization patterns, tax	(0.5)
Adjustments for assumed changes in policyholder liabilities, tax	(1.1)	(0.3)
Other comprehensive income (loss), tax	4.2	0.5
Other comprehensive income (loss) before reclassifications, after-tax	(16.6)	0.6
Reclassification from accumulated other comprehensive income, after-tax	(5.4)	(3.5)
Adjustments for assumed changes in amortization patterns, after-tax	1.8
Adjustments for assumed changes in policyholder liabilities, after-tax	3.9	0.7
Other comprehensive income (loss)	(16.3)	(2.2)
Foreign currency translation adjustment including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	154.9	(56.1)
Other comprehensive income (loss), tax	(1.5)	(0.1)
Other comprehensive income (loss)	153.4	(56.2)
Unrecognized postretirement benefit obligation including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax		(9.1)
Other comprehensive income (loss), pre-tax	9.0	3.9
Other comprehensive income (loss) before reclassifications, tax		1.9
Reclassification from accumulated other comprehensive income, tax	(2.3)	(3.5)
Other comprehensive income (loss), tax	(2.3)	(1.6)
Other comprehensive income (loss) before reclassifications, after-tax		(7.2)
Reclassification from accumulated other comprehensive income, after-tax	6.7	9.5
Other comprehensive income (loss)	6.7	2.3
Unrecognized postretirement benefit obligation including NCI | Operating expense
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax	$ 9.0	$ 13.0