Stockholders' Equity - Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other Comprehensive Income (Loss)
Other comprehensive income (loss), pre-tax	$ 1,683.6	$ 3,326.3	$ (2,258.6)
Other comprehensive income (loss), tax	(336.6)	(719.6)	422.9
Other comprehensive income (loss)	1,347.0	2,606.7	(1,835.7)
Net unrealized gains (losses) on available-for-sale securities including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	3,441.4	4,414.1	(2,517.5)
Adjustments for assumed changes in amortization patterns, pre-tax	(179.0)	(293.0)	185.9
Adjustments for assumed changes in policyholder liabilities, pre-tax	(1,478.2)	(847.7)	346.9
Other comprehensive income (loss), pre-tax	1,731.3	3,320.8	(1,928.3)
Other comprehensive income (loss) before reclassifications, tax	(728.7)	(947.0)	515.4
Reclassification from accumulated other comprehensive income, tax	12.6	(9.7)	(5.3)
Adjustments for assumed changes in amortization patterns, tax	37.6	61.5	(39.1)
Adjustments for assumed changes in policyholder liabilities, tax	323.7	190.5	(72.3)
Other comprehensive income (loss), tax	(354.8)	(704.7)	398.7
Other comprehensive income (loss) before reclassifications, after-tax	2,712.7	3,467.1	(2,002.1)
Reclassification from accumulated other comprehensive income, after-tax	(40.3)	37.7	51.1
Adjustments for assumed changes in amortization patterns, after-tax	(141.4)	(231.5)	146.8
Adjustments for assumed changes in policyholder liabilities, after-tax	(1,154.5)	(657.2)	274.6
Other comprehensive income (loss)	1,376.5	2,616.1	(1,529.6)
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	(52.9)	47.4	56.4
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		5.2	39.7
Adjustments for assumed changes in amortization patterns, pre-tax		(1.4)	(5.3)
Adjustments for assumed changes in policyholder liabilities, pre-tax			(0.8)
Other comprehensive income (loss), pre-tax		3.8	33.6
Other comprehensive income (loss) before reclassifications, tax		(1.1)	(8.4)
Adjustments for assumed changes in amortization patterns, tax		0.3	1.1
Adjustments for assumed changes in policyholder liabilities, tax			0.1
Other comprehensive income (loss), tax		(0.8)	(7.2)
Other comprehensive income (loss) before reclassifications, after-tax		4.1	31.3
Adjustments for assumed changes in amortization patterns, after-tax		(1.1)	(4.2)
Adjustments for assumed changes in policyholder liabilities, after-tax			(0.7)
Other comprehensive income (loss)		3.0	26.4
Net unrealized gains (losses) on derivative instruments including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	(28.1)	(0.5)	50.4
Reclassification from accumulated other comprehensive income, pre-tax	(27.1)	(23.9)	(40.1)
Adjustments for assumed changes in amortization patterns, pre-tax	2.7	3.1	0.3
Adjustments for assumed changes in policyholder liabilities, pre-tax	7.8	7.9	5.7
Other comprehensive income (loss), pre-tax	(44.7)	(13.4)	16.3
Other comprehensive income (loss) before reclassifications, tax	6.5		(3.4)
Reclassification from accumulated other comprehensive income, tax	5.1	4.9	4.2
Adjustments for assumed changes in amortization patterns, tax	(0.5)	(0.6)	(0.1)
Adjustments for assumed changes in policyholder liabilities, tax	(1.6)	(1.9)	(1.0)
Other comprehensive income (loss), tax	9.5	2.4	(0.3)
Other comprehensive income (loss) before reclassifications, after-tax	(21.6)	(0.5)	47.0
Reclassification from accumulated other comprehensive income, after-tax	(22.0)	(19.0)	(35.9)
Adjustments for assumed changes in amortization patterns, after-tax	2.2	2.5	0.2
Adjustments for assumed changes in policyholder liabilities, after-tax	6.2	6.0	4.7
Other comprehensive income (loss)	(35.2)	(11.0)	16.0
Foreign currency translation adjustment including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	(11.7)	(112.3)
Other comprehensive income (loss), pre-tax	31.3	(86.2)	(303.9)
Other comprehensive income (loss) before reclassifications, tax	(2.6)	7.5
Reclassification from accumulated other comprehensive income, tax	1.9
Other comprehensive income (loss), tax	(0.7)	7.5	16.3
Other comprehensive income (loss) before reclassifications, after-tax	(14.3)	(104.8)
Reclassification from accumulated other comprehensive income, after-tax	44.9	26.1
Other comprehensive income (loss)	30.6	(78.7)	(287.6)
Foreign currency translation adjustment including NCI | Net realized capital gains (losses)
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax	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	5.7
Unrecognized postretirement benefit obligation including NCI
Other Comprehensive Income (Loss)
Other comprehensive income (loss) before reclassifications, pre-tax	(91.9)	43.6	(125.3)
Other comprehensive income (loss), pre-tax	(34.3)	101.3	(76.3)
Other comprehensive income (loss) before reclassifications, tax	24.9	(8.6)	29.4
Reclassification from accumulated other comprehensive income, tax	(15.5)	(15.4)	(14.0)
Other comprehensive income (loss), tax	9.4	(24.0)	15.4
Other comprehensive income (loss) before reclassifications, after-tax	(67.0)	35.0	(95.9)
Reclassification from accumulated other comprehensive income, after-tax	42.1	42.3	35.0
Other comprehensive income (loss)	(24.9)	77.3	(60.9)
Unrecognized postretirement benefit obligation including NCI | Operating expense
Other Comprehensive Income (Loss)
Reclassification from accumulated other comprehensive income, pre-tax	$ 57.6	$ 57.7	$ 49.0