Stockholders' Equity - Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	$ (189.7)	$ 357.7	$ (909.0)	$ 939.2
Other comprehensive income (loss), tax	2.8	(51.3)	142.4	(231.0)
Other comprehensive income (loss)	(186.9)	306.4	(766.6)	708.2
Net unrealized gains (losses) on available-for-sale securities including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	(546.4)	960.1	(2,461.6)	2,378.1
Adjustments for assumed changes in amortization patterns, pre-tax	32.2	(109.0)	121.3	(112.9)
Adjustments for assumed changes in policyholder liabilities, pre-tax	477.2	(565.7)	1,573.7	(1,136.0)
Other comprehensive income (loss), pre-tax	(39.6)	288.3	(770.4)	1,074.7
Other comprehensive income (loss) before reclassifications, tax	127.4	(200.0)	554.9	(500.1)
Reclassification from accumulated other comprehensive income, tax	0.2	(1.0)	0.2	12.6
Adjustments for assumed changes in amortization patterns, tax	(6.8)	22.9	(25.5)	23.7
Adjustments for assumed changes in policyholder liabilities, tax	(104.8)	116.5	(365.3)	241.9
Other comprehensive income (loss), tax	16.0	(61.6)	164.3	(221.9)
Other comprehensive income (loss) before reclassifications, after-tax	(419.0)	760.1	(1,906.7)	1,878.0
Reclassification from accumulated other comprehensive income, after-tax	(2.4)	1.9	(3.6)	(41.9)
Adjustments for assumed changes in amortization patterns, after-tax	25.4	(86.1)	95.8	(89.2)
Adjustments for assumed changes in policyholder liabilities, after-tax	372.4	(449.2)	1,208.4	(894.1)
Other comprehensive income (loss)	(23.6)	226.7	(606.1)	852.8
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	(2.6)	2.9	(3.8)	(54.5)
Net unrealized gains (losses) on derivative instruments including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	31.2	(29.2)	53.7	26.2
Reclassification from accumulated other comprehensive income, pre-tax	(3.9)	(4.6)	(13.0)	(22.8)
Adjustments for assumed changes in amortization patterns, pre-tax	(0.5)	0.2	(0.5)	0.9
Adjustments for assumed changes in policyholder liabilities, pre-tax	(0.4)	1.7	1.0	4.6
Other comprehensive income (loss), pre-tax	26.4	(31.9)	41.2	8.9
Other comprehensive income (loss) before reclassifications, tax	(6.6)	6.2	(11.3)	(4.9)
Reclassification from accumulated other comprehensive income, tax	0.8	0.9	2.7	4.2
Adjustments for assumed changes in amortization patterns, tax	0.1		0.1	(0.2)
Adjustments for assumed changes in policyholder liabilities, tax		(0.3)	(0.3)	(0.9)
Other comprehensive income (loss), tax	(5.7)	6.8	(8.8)	(1.8)
Other comprehensive income (loss) before reclassifications, after-tax	24.6	(23.0)	42.4	21.3
Reclassification from accumulated other comprehensive income, after-tax	(3.1)	(3.7)	(10.3)	(18.6)
Adjustments for assumed changes in amortization patterns, after-tax	(0.4)	0.2	(0.4)	0.7
Adjustments for assumed changes in policyholder liabilities, after-tax	(0.4)	1.4	0.7	3.7
Other comprehensive income (loss)	20.7	(25.1)	32.4	7.1
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		8.7
Foreign currency translation adjustment including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax		75.5		(231.1)
Other comprehensive income (loss), pre-tax	(189.3)	87.0	(209.5)	(188.1)
Other comprehensive income (loss) before reclassifications, tax		5.5		2.7
Reclassification from accumulated other comprehensive income, tax		1.8		1.8
Other comprehensive income (loss), tax	(4.0)	7.3	(4.6)	4.5
Other comprehensive income (loss) before reclassifications, after-tax		81.0		(228.4)
Reclassification from accumulated other comprehensive income, after-tax		13.3		44.8
Other comprehensive income (loss)	(193.3)	94.3	(214.1)	(183.6)
Foreign currency translation adjustment including NCI | Net realized capital gains (losses)
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax		11.5		43.0
Unrecognized postretirement benefit obligation including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax			(9.1)	0.6
Other comprehensive income (loss), pre-tax	12.8	14.3	29.7	43.7
Other comprehensive income (loss) before reclassifications, tax			1.9	(0.2)
Reclassification from accumulated other comprehensive income, tax	(3.5)	(3.8)	(10.4)	(11.6)
Other comprehensive income (loss), tax	(3.5)	(3.8)	(8.5)	(11.8)
Other comprehensive income (loss) before reclassifications, after-tax			(7.2)	0.4
Reclassification from accumulated other comprehensive income, after-tax	9.3	10.5	28.4	31.5
Other comprehensive income (loss)	9.3	10.5	21.2	31.9
Unrecognized postretirement benefit obligation including NCI | Operating expense
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax	$ 12.8	$ 14.3	$ 38.8	$ 43.1