Employee Benefit Arrangements - Level 3 Plan Assets (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, ending balance	$ 81.8
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, beginning balance			1,371.7
Fair value of plan assets, ending balance	1,657.7	1,556.8	1,371.7
Pension Plan | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, beginning balance	222.1	190.4
Actual Return on Plan Assets, Held at Year-end	4.0	14.5
Actual Return on Plan Assets, Sold During Year	9.2	2.5
Purchases	17.0	24.6
Issuances	0	0
Sales	(23.2)	(9.9)
Settlements	0	0
Transfers in to Level 3	0	0
Transfers out of Level 3	0	0
Fair value of plan assets, ending balance	229.1	222.1
Pension Plan | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, ending balance	81.8	74.6
Pension Plan | Real estate | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, beginning balance	74.6	67.4
Actual Return on Plan Assets, Held at Year-end	0.3	0.8
Actual Return on Plan Assets, Sold During Year	0	0
Purchases	6.9	6.4
Issuances	0	0
Sales	0	0
Settlements	0	0
Transfers in to Level 3	0	0
Transfers out of Level 3	0	0
Fair value of plan assets, ending balance	81.8	74.6
Pension Plan | Limited partnerships | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, beginning balance	147.5	123.0
Actual Return on Plan Assets, Held at Year-end	3.7	13.7
Actual Return on Plan Assets, Sold During Year	9.2	2.5
Purchases	10.1	18.2
Issuances	0	0
Sales	(23.2)	(9.9)
Settlements	0	0
Transfers in to Level 3	0	0
Transfers out of Level 3	0	0
Fair value of plan assets, ending balance	$ 147.3	$ 147.5