Segments (Segment Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums	$ 515.8	$ 507.3	$ 519.2	$ 510.1	$ 528.6	$ 524.4	$ 535.1	$ 536.2	$ 2,052.4	$ 2,124.3	$ 2,163.9
Administrative fees	33.8	32.8	32.6	31.6	32.0	30.5	30.4	28.7	130.8	121.6	114.7
Net investment income	153.4	147.1	149.8	150.6	160.8	155.8	154.1	159.2	600.9	629.9	628.5
Net capital losses	(7.2)	(5.2)	(0.5)	(1.1)	(2.8)	(8.7)	(2.6)	(1.6)	(14.0)	(15.7)	(8.7)
Total revenues	695.8	682.0	701.1	691.2	718.6	702.0	717.0	722.5	2,770.1	2,860.1	2,898.4
Benefits to policyholders	394.1	360.8	426.2	401.1	394.5	395.0	424.1	441.8	1,582.2	1,655.4	1,793.0
Interest credited									164.6	176.4	171.3
Operating expenses									470.6	442.6	470.5
Commissions and bonuses									207.2	207.9	203.7
Premium taxes									31.5	35.3	37.5
Interest expense									31.8	34.4	39.9
Net increase in deferred acquisition costs, value of business acquired and other intangible assets									(4.7)	(4.9)	(0.9)
Total benefits and expenses									2,483.2	2,547.1	2,715.0
Income (loss) before income taxes									286.9	313.0	183.4
Total assets	22,729.9				21,393.3				22,729.9	21,393.3	19,791.3
Insurance Services [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums									2,041.9	2,117.4	2,157.2
Administrative fees									16.7	15.3	13.9
Net investment income									299.5	320.6	339.7
Net capital losses										0	0
Total revenues									2,358.1	2,453.3	2,510.8
Benefits to policyholders									1,561.4	1,637.7	1,773.3
Interest credited									2.6	4.3	4.2
Operating expenses									349.2	340.0	349.1
Commissions and bonuses									170.6	174.2	172.7
Premium taxes									31.5	35.2	37.5
Net increase in deferred acquisition costs, value of business acquired and other intangible assets									(11.9)	(10.5)	(5.8)
Total benefits and expenses									2,103.4	2,180.9	2,331.0
Income (loss) before income taxes									254.7	272.4	179.8
Total assets	8,352.0				8,088.1				8,352.0	8,088.1	8,250.8
Employee Benefits [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums									1,842.8	1,926.9	1,980.6
Administrative fees									16.5	15.1	13.7
Net investment income									247.4	267.8	286.0
Net capital losses										0	0
Total revenues									2,106.7	2,209.8	2,280.3
Benefits to policyholders									1,431.4	1,515.9	1,657.1
Interest credited									2.6	4.3	4.2
Operating expenses									321.4	313.3	323.6
Commissions and bonuses									122.6	126.7	125.1
Premium taxes									27.9	31.3	33.6
Net increase in deferred acquisition costs, value of business acquired and other intangible assets									(1.0)	1.9	6.9
Total benefits and expenses									1,904.9	1,993.4	2,150.5
Income (loss) before income taxes									201.8	216.4	129.8
Total assets	5,952.5				5,744.7				5,952.5	5,744.7	5,991.4
Individual Disability [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums									199.1	190.5	176.6
Administrative fees									0.2	0.2	0.2
Net investment income									52.1	52.8	53.7
Net capital losses										0	0
Total revenues									251.4	243.5	230.5
Benefits to policyholders									130.0	121.8	116.2
Interest credited										0	0
Operating expenses									27.8	26.7	25.5
Commissions and bonuses									48.0	47.5	47.6
Premium taxes									3.6	3.9	3.9
Net increase in deferred acquisition costs, value of business acquired and other intangible assets									(10.9)	(12.4)	(12.7)
Total benefits and expenses									198.5	187.5	180.5
Income (loss) before income taxes									52.9	56.0	50.0
Total assets	2,399.5				2,343.4				2,399.5	2,343.4	2,259.4
Asset Management Segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums									10.5	6.9	6.7
Administrative fees									133.4	124.9	118.8
Net investment income									283.3	293.2	278.6
Net capital losses										0	0
Total revenues									427.2	425.0	404.1
Benefits to policyholders									20.8	17.7	19.7
Interest credited									162.0	172.1	167.1
Operating expenses									122.7	116.5	117.4
Commissions and bonuses									36.6	33.7	31.0
Premium taxes									0	0.1	0
Net increase in deferred acquisition costs, value of business acquired and other intangible assets									7.2	5.6	4.9
Total benefits and expenses									349.3	345.7	340.1
Income (loss) before income taxes									77.9	79.3	64.0
Total assets	14,025.6				12,777.1				14,025.6	12,777.1	11,046.0
Other Category [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums									0	0	0
Administrative fees									(19.3)	(18.6)	(18.0)
Net investment income									18.1	16.1	10.2
Net capital losses									(14.0)	(15.7)	(8.7)
Total revenues									(15.2)	(18.2)	(16.5)
Operating expenses									(1.3)	(13.9)	4.0
Interest expense									31.8	34.4	39.9
Total benefits and expenses									30.5	20.5	43.9
Income (loss) before income taxes									(45.7)	(38.7)	(60.4)
Total assets	$ 352.3				$ 528.1				$ 352.3	$ 528.1	$ 494.5