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Stockholders' Equity - Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	$ 1,079.8	$ 2,602.6	$ (719.3)	$ 581.5
Other comprehensive income (loss), tax	(230.0)	(554.5)	139.6	(179.7)
Other comprehensive income (loss)	849.8	2,048.1	(579.7)	401.8
Net unrealized gains (losses) on available-for-sale securities including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	1,361.5	4,195.7	(1,915.2)	1,418.0
Adjustments for assumed changes in amortization patterns, pre-tax	(55.5)	(336.4)	89.1	(3.9)
Adjustments for assumed changes in policyholder liabilities, pre-tax	(318.8)	(1,232.5)	1,096.5	(570.3)
Other comprehensive income (loss), pre-tax	1,013.4	2,578.9	(730.8)	786.4
Other comprehensive income (loss) before reclassifications, tax	(273.0)	(900.4)	427.5	(300.1)
Reclassification from accumulated other comprehensive income, tax	(5.7)	11.0		13.6
Adjustments for assumed changes in amortization patterns, tax	11.7	70.6	(18.7)	0.8
Adjustments for assumed changes in policyholder liabilities, tax	44.5	277.6	(260.5)	125.4
Other comprehensive income (loss), tax	(222.5)	(541.2)	148.3	(160.3)
Other comprehensive income (loss) before reclassifications, after-tax	1,088.5	3,295.3	(1,487.7)	1,117.9
Reclassification from accumulated other comprehensive income, after-tax	20.5	(36.9)	(1.2)	(43.8)
Adjustments for assumed changes in amortization patterns, after-tax	(43.8)	(265.8)	70.4	(3.1)
Adjustments for assumed changes in policyholder liabilities, after-tax	(274.3)	(954.9)	836.0	(444.9)
Other comprehensive income (loss)	790.9	2,037.7	(582.5)	626.1
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	26.2	(47.9)	(1.2)	(57.4)
Net unrealized gains (losses) on derivative instruments including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	21.8	(10.8)	22.5	55.4
Reclassification from accumulated other comprehensive income, pre-tax	(4.7)	(12.6)	(9.1)	(18.2)
Adjustments for assumed changes in amortization patterns, pre-tax		1.0		0.7
Adjustments for assumed changes in policyholder liabilities, pre-tax	0.4	1.7	1.4	2.9
Other comprehensive income (loss), pre-tax	17.5	(20.7)	14.8	40.8
Other comprehensive income (loss) before reclassifications, tax	(4.6)	3.1	(4.7)	(11.1)
Reclassification from accumulated other comprehensive income, tax	1.0	1.8	1.9	3.3
Adjustments for assumed changes in amortization patterns, tax		(0.2)		(0.2)
Adjustments for assumed changes in policyholder liabilities, tax		(0.4)	(0.3)	(0.6)
Other comprehensive income (loss), tax	(3.6)	4.3	(3.1)	(8.6)
Other comprehensive income (loss) before reclassifications, after-tax	17.2	(7.7)	17.8	44.3
Reclassification from accumulated other comprehensive income, after-tax	(3.7)	(10.8)	(7.2)	(14.9)
Adjustments for assumed changes in amortization patterns, after-tax		0.8		0.5
Adjustments for assumed changes in policyholder liabilities, after-tax	0.4	1.3	1.1	2.3
Other comprehensive income (loss)	13.9	(16.4)	11.7	32.2
Foreign currency translation adjustment including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax		30.0		(306.6)
Other comprehensive income (loss), pre-tax	35.9	30.0	(20.2)	(275.1)
Other comprehensive income (loss) before reclassifications, tax		(13.6)		(2.8)
Other comprehensive income (loss), tax	(0.5)	(13.6)	(0.6)	(2.8)
Other comprehensive income (loss) before reclassifications, after-tax		16.4		(309.4)
Reclassification from accumulated other comprehensive income, after-tax				31.5
Other comprehensive income (loss)	35.4	16.4	(20.8)	(277.9)
Foreign currency translation adjustment including NCI | Net realized capital gains (losses)
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax				31.5
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	13.0	14.4	16.9	29.4
Other comprehensive income (loss) before reclassifications, tax		(0.1)	1.9	(0.2)
Reclassification from accumulated other comprehensive income, tax	(3.4)	(3.9)	(6.9)	(7.8)
Other comprehensive income (loss), tax	(3.4)	(4.0)	(5.0)	(8.0)
Other comprehensive income (loss) before reclassifications, after-tax		(0.1)	(7.2)	0.4
Reclassification from accumulated other comprehensive income, after-tax	9.6	10.5	19.1	21.0
Other comprehensive income (loss)	9.6	10.4	11.9	21.4
Unrecognized postretirement benefit obligation including NCI | Operating expense
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax	$ 13.0	$ 14.4	$ 26.0	$ 28.8

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