Supplemental Schedule of Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums, Life Insurance in Force	$ 1,316.0	$ 1,363.0	$ 1,462.0
Ceded Premiums, Life Insurance in Force	192,864.0	186,172.0	205,529.0
Assumed Premiums, Life Insurance in Force	3,480,206.0	3,329,181.0	3,297,275.0
Premiums, Net, Life Insurance in Force	$ 3,288,658.0	$ 3,144,372.0	$ 3,093,208.0
Life Insurance in Force Premiums, Percentage Assumed to Net	105.80%	105.90%	106.60%
Direct Premiums Earned	$ 76.1	$ 62.5	$ 61.6
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	928.6	860.0	862.9
Assumed Premiums Earned	12,149.7	11,341.3	10,642.4
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 11,297.2	$ 10,543.8	$ 9,841.1
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	107.50%	107.60%	108.10%
US Traditional Segment Reporting Information [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Earned	$ 29.4	$ 31.6	$ 30.5
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	590.8	593.7	610.4
Assumed Premiums Earned	6,291.0	6,095.4	5,936.2
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 5,729.6	$ 5,533.3	$ 5,356.3
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	109.80%	110.20%	110.80%
US Non-Traditional Segment Reporting Information [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Earned	$ 1.8	$ 4.7	$ 4.3
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	0.0	0.0	0.0
Assumed Premiums Earned	37.0	22.5	19.4
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 38.8	$ 27.2	$ 23.7
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	95.40%	82.70%	81.90%
Canada Traditional Segment Reporting Information [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Earned	$ 0.0	$ 0.0	$ 0.0
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	53.8	46.9	38.1
Assumed Premiums Earned	1,119.5	1,070.9	940.1
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 1,065.7	$ 1,024.0	$ 902.0
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	105.00%	104.60%	104.20%
Canada Non-Traditional Segment Reporting Information [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Earned	$ 0.0	$ 0.0	$ 0.0
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	0.0	0.0	0.0
Assumed Premiums Earned	88.9	43.4	38.2
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 88.9	$ 43.4	$ 38.2
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	100.00%	100.00%	100.00%
EMEA Traditional Segment Reporting Information [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Earned	$ 44.7	$ 26.0	$ 26.6
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	52.7	25.9	34.9
Assumed Premiums Earned	1,449.4	1,423.1	1,310.0
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 1,441.4	$ 1,423.2	$ 1,301.7
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	100.60%	100.00%	100.60%
EMEA Non-Traditional Segment Reporting Information [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Earned	$ 0.2	$ 0.2	$ 0.2
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	148.4	143.7	125.0
Assumed Premiums Earned	366.4	338.8	288.5
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 218.2	$ 195.3	$ 163.7
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	167.90%	173.50%	176.20%
Asia Pacific Traditional Segment Reporting Information [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Earned	$ 0.0	$ 0.0	$ 0.0
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	82.9	49.8	54.5
Assumed Premiums Earned	2,651.5	2,346.2	2,107.5
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 2,568.6	$ 2,296.4	$ 2,053.0
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	103.20%	102.20%	102.70%
Asia Pacific Non-Traditional Segment Reporting Information [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Earned	$ 0.0	$ 0.0	$ 0.0
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	0.0	0.0	0.0
Assumed Premiums Earned	146.0	0.9	2.4
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 146.0	$ 0.9	$ 2.4
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	100.00%	100.00%	100.00%
Corporate And Other Segment Reporting Information [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Earned		$ 0.0	$ 0.0
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies		0.0	0.0
Assumed Premiums Earned		0.1	0.1
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount		$ 0.1	$ 0.1
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net		100.00%	100.00%