Investments - Loans by U.S. Region (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 4,030.1	$ 3,730.3
Loans by region percentage of total loans	100.00%	100.00%
Pacific
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 931.8	$ 867.5
Loans by region percentage of total loans	23.10%	23.30%
South Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 912.6	$ 857.3
Loans by region percentage of total loans	22.70%	23.00%
West South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 407.9	$ 414.8
Loans by region percentage of total loans	10.10%	11.10%
Middle Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 674.3	$ 556.1
Loans by region percentage of total loans	16.70%	14.90%
East North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 428.3	$ 380.8
Loans by region percentage of total loans	10.60%	10.20%
Mountain
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 315.7	$ 304.1
Loans by region percentage of total loans	7.90%	8.20%
West North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 222.9	$ 208.6
Loans by region percentage of total loans	5.50%	5.60%
East South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 61.7	$ 59.7
Loans by region percentage of total loans	1.50%	1.50%
New England
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 74.9	$ 81.4
Loans by region percentage of total loans	1.90%	2.20%