Investments (excluding Consolidated Investment Entities) - Loans by U.S. Region (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 10,450.7	$ 9,796.9
Percentage of Total	100.00%	100.00%
Pacific
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 2,605.3	$ 2,395.9
Percentage of Total	24.90%	24.60%
South Atlantic
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 2,318.9	$ 2,028.0
Percentage of Total	22.20%	20.70%
West South Central
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 1,103.7	$ 1,147.7
Percentage of Total	10.60%	11.70%
Middle Atlantic
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 1,499.1	$ 1,402.0
Percentage of Total	14.30%	14.30%
East North Central
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 1,103.3	$ 1,030.8
Percentage of Total	10.60%	10.50%
Mountain
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 924.2	$ 832.2
Percentage of Total	8.80%	8.50%
West North Central
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 488.8	$ 514.0
Percentage of Total	4.70%	5.20%
New England
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 222.8	$ 197.0
Percentage of Total	2.10%	2.00%
East South Central
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 184.6	$ 249.3
Percentage of Total	1.80%	2.50%