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Stockholders' Equity - Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	$ 357.7	$ 863.9	$ 939.2	$ 3,678.0
Other comprehensive income (loss), tax	(51.3)	(207.6)	(231.0)	(802.7)
Other comprehensive income (loss)	306.4	656.3	708.2	2,875.3
Net unrealized gains (losses) on available-for-sale securities including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	960.1	1,391.9	2,378.1	5,012.9
Adjustments for assumed changes in amortization patterns, pre-tax	(109.0)	(60.9)	(112.9)	(300.6)
Adjustments for assumed changes in policyholder liabilities, pre-tax	(565.7)	(342.3)	(1,136.0)	(1,104.9)
Other comprehensive income (loss), pre-tax	288.3	991.8	1,074.7	3,647.1
Other comprehensive income (loss) before reclassifications, tax	(200.0)	(301.5)	(500.1)	(1,088.9)
Reclassification from accumulated other comprehensive income, tax	(1.0)	(0.4)	12.6	(7.6)
Adjustments for assumed changes in amortization patterns, tax	22.9	12.7	23.7	63.1
Adjustments for assumed changes in policyholder liabilities, tax	116.5	84.5	241.9	262.6
Other comprehensive income (loss), tax	(61.6)	(204.7)	(221.9)	(770.8)
Other comprehensive income (loss) before reclassifications, after-tax	760.1	1,090.4	1,878.0	3,924.0
Reclassification from accumulated other comprehensive income, after-tax	1.9	2.7	(41.9)	32.1
Adjustments for assumed changes in amortization patterns, after-tax	(86.1)	(48.2)	(89.2)	(237.5)
Adjustments for assumed changes in policyholder liabilities, after-tax	(449.2)	(257.8)	(894.1)	(842.3)
Other comprehensive income (loss)	226.7	787.1	852.8	2,876.3
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	2.9	3.1	(54.5)	39.7
Noncredit component of impairment losses on fixed maturities available-for-sale including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax		6.4		3.9
Adjustments for assumed changes in amortization patterns, pre-tax		(0.8)		(2.0)
Adjustments for assumed changes in policyholder liabilities, pre-tax		(0.2)		(0.1)
Other comprehensive income (loss), pre-tax		5.4		1.8
Other comprehensive income (loss) before reclassifications, tax		(1.4)		(0.8)
Adjustments for assumed changes in amortization patterns, tax		0.2		0.4
Adjustments for assumed changes in policyholder liabilities, tax		0.1		0.1
Other comprehensive income (loss), tax		(1.1)		(0.3)
Other comprehensive income (loss) before reclassifications, after-tax		5.0		3.1
Adjustments for assumed changes in amortization patterns, after-tax		(0.6)		(1.6)
Adjustments for assumed changes in policyholder liabilities, after-tax		(0.1)
Other comprehensive income (loss)		4.3		1.5
Net unrealized gains (losses) on derivative instruments including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	(29.2)	19.9	26.2	24.2
Reclassification from accumulated other comprehensive income, pre-tax	(4.6)	(4.9)	(22.8)	(19.7)
Adjustments for assumed changes in amortization patterns, pre-tax	0.2	(0.1)	0.9	2.4
Adjustments for assumed changes in policyholder liabilities, pre-tax	1.7	1.2	4.6	6.0
Other comprehensive income (loss), pre-tax	(31.9)	16.1	8.9	12.9
Other comprehensive income (loss) before reclassifications, tax	6.2	(4.3)	(4.9)	(5.2)
Reclassification from accumulated other comprehensive income, tax	0.9	1.1	4.2	3.9
Adjustments for assumed changes in amortization patterns, tax			(0.2)	(0.5)
Adjustments for assumed changes in policyholder liabilities, tax	(0.3)	(0.2)	(0.9)	(1.3)
Other comprehensive income (loss), tax	6.8	(3.4)	(1.8)	(3.1)
Other comprehensive income (loss) before reclassifications, after-tax	(23.0)	15.6	21.3	19.0
Reclassification from accumulated other comprehensive income, after-tax	(3.7)	(3.8)	(18.6)	(15.8)
Adjustments for assumed changes in amortization patterns, after-tax	0.2	(0.1)	0.7	1.9
Adjustments for assumed changes in policyholder liabilities, after-tax	1.4	1.0	3.7	4.7
Other comprehensive income (loss)	(25.1)	12.7	7.1	9.8
Foreign currency translation adjustment including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	75.5	(163.0)	(231.1)	(117.5)
Other comprehensive income (loss), pre-tax	87.0	(163.0)	(188.1)	(91.4)
Other comprehensive income (loss) before reclassifications, tax	5.5	5.2	2.7	0.4
Reclassification from accumulated other comprehensive income, tax	1.8		1.8
Other comprehensive income (loss), tax	7.3	5.2	4.5	0.4
Other comprehensive income (loss) before reclassifications, after-tax	81.0	(157.8)	(228.4)	(117.1)
Reclassification from accumulated other comprehensive income, after-tax	13.3		44.8	26.1
Other comprehensive income (loss)	94.3	(157.8)	(183.6)	(91.0)
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	26.1
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	5.7
Unrecognized postretirement benefit obligation including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax			0.6	63.4
Other comprehensive income (loss), pre-tax	14.3	13.6	43.7	107.6
Other comprehensive income (loss) before reclassifications, tax			(0.2)	(17.1)
Reclassification from accumulated other comprehensive income, tax	(3.8)	(3.6)	(11.6)	(11.8)
Other comprehensive income (loss), tax	(3.8)	(3.6)	(11.8)	(28.9)
Other comprehensive income (loss) before reclassifications, after-tax			0.4	46.3
Reclassification from accumulated other comprehensive income, after-tax	10.5	10.0	31.5	32.4
Other comprehensive income (loss)	10.5	10.0	31.9	78.7
Unrecognized postretirement benefit obligation including NCI | Operating expense
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax	$ 14.3	$ 13.6	$ 43.1	$ 44.2

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