Investments - Loans by U.S. Region (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 9,953.1	[1]	$ 9,316.0	[1]
Loans by region percentage of total loans	100.00%		100.00%
Pacific
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	2,490.9	[1]	2,281.8	[1]
Loans by region percentage of total loans	25.00%		24.50%
South Atlantic
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	2,031.3	[1]	1,936.0	[1]
Loans by region percentage of total loans	20.40%		20.80%
Middle Atlantic
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	1,355.1	[1]	1,112.0	[1]
Loans by region percentage of total loans	13.60%		11.90%
West South Central
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	1,101.6	[1]	1,122.3	[1]
Loans by region percentage of total loans	11.10%		12.00%
East North Central
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	1,190.5	[1]	1,037.5
Loans by region percentage of total loans	12.00%		11.10%
Mountain
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	837.8	[1]	790.4	[1]
Loans by region percentage of total loans	8.40%		8.50%
West North Central
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	528.4	[1]	517.2	[1]
Loans by region percentage of total loans	5.30%		5.60%
East South Central
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	235.2	[1]	200.7	[1]
Loans by region percentage of total loans	2.40%		2.20%
New England
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 182.3	[1]	$ 318.1
Loans by region percentage of total loans	1.80%		3.40%

[1] Balances do not include collective valuation allowance for losses.