Investments - Loans by U.S. Region (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 3,868.6	$ 3,514.1
Loans by region percentage of total loans	100.00%	100.00%
Pacific
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 921.1	$ 802.6
Loans by region percentage of total loans	23.80%	22.80%
South Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 854.8	$ 746.5
Loans by region percentage of total loans	22.10%	21.20%
West South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 464.8	$ 448.4
Loans by region percentage of total loans	12.00%	12.80%
Middle Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 563.9	$ 505.8
Loans by region percentage of total loans	14.60%	14.40%
East North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 406.2	$ 355.3
Loans by region percentage of total loans	10.50%	10.10%
Mountain
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 300.9	$ 274.0
Loans by region percentage of total loans	7.80%	7.80%
West North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 212.9	$ 219.6
Loans by region percentage of total loans	5.50%	6.30%
East South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 60.4	$ 87.1
Loans by region percentage of total loans	1.50%	2.50%
New England
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 83.6	$ 74.8
Loans by region percentage of total loans	2.20%	2.10%