Schedule 4 - Reinsurance Schedule (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums, Life Insurance in Force	$ 19,208.7				$ 20,889.0				$ 19,208.7	$ 20,889.0	$ 20,209.8
Direct Premiums Earned									2,295.3	2,171.7	1,970.1
Ceded Premiums, Life Insurance in Force	455.2				486.9				455.2	486.9	514.2
Ceded to Other Companies									18.2	19.0	20.6
Assumed Premiums, Life Insurance in Force	184.0				195.0				184.0	195.0	205.7
Assumed Premiums Earned									72.9	67.3	60.1
Premiums, Net, Life Insurance in Force	18,937.5				20,597.1				18,937.5	20,597.1	19,901.3
Net Amount	$ 605.9	$ 598.2	$ 582.5	$ 563.4	$ 561.4	$ 558.9	$ 553.7	$ 546.0	$ 2,350.0	$ 2,220.0	$ 2,009.6
Life Insurance in Force Premiums, Percentage Assumed to Net	1.00%				0.90%				1.00%	0.90%	1.00%
Premiums, Percentage Assumed to Net									3.10%	3.00%	3.00%
Life Insurance Product Line [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums Earned									$ 366.8	$ 381.4	$ 374.1
Ceded to Other Companies									1.4	1.4	1.4
Assumed Premiums Earned									1.1	1.3	1.4
Net Amount									$ 366.5	$ 381.3	$ 374.1
Premiums, Percentage Assumed to Net									0.30%	0.30%	0.40%
Accident and Health Insurance [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums Earned									$ 171.0	$ 144.9	$ 139.8
Ceded to Other Companies									1.5	0.5	0.5
Assumed Premiums Earned									5.4	5.3	5.6
Net Amount									$ 174.9	$ 149.7	$ 144.9
Premiums, Percentage Assumed to Net									3.10%	3.50%	3.90%
Property, Liability and Casualty Insurance [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums Earned									$ 1,757.5	$ 1,645.4	$ 1,456.2
Ceded to Other Companies									15.3	17.1	18.7
Assumed Premiums Earned									66.4	60.7	53.1
Net Amount									$ 1,808.6	$ 1,689.0	$ 1,490.6
Premiums, Percentage Assumed to Net									3.70%	3.60%	3.60%