Investments - Loans by U.S. Region (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 3,284.2	[1]	$ 2,874.0	[1]
Loans by region percentage of total loans	100.00%	[1]	100.00%	[1]
Pacific
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	651.0	[1]	564.1	[1]
Loans by region percentage of total loans	19.80%	[1]	19.60%	[1]
South Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	679.9	[1]	561.0	[1]
Loans by region percentage of total loans	20.70%	[1]	19.50%	[1]
Middle Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	407.6	[1]	332.7	[1]
Loans by region percentage of total loans	12.40%	[1]	11.60%	[1]
East North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	376.5	[1]	337.8	[1]
Loans by region percentage of total loans	11.50%	[1]	11.80%	[1]
West South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	501.3	[1]	460.4	[1]
Loans by region percentage of total loans	15.30%	[1]	16.00%	[1]
Mountain
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	250.1	[1]	214.5	[1]
Loans by region percentage of total loans	7.60%	[1]	7.50%	[1]
New England
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	116.2	[1]	119.1	[1]
Loans by region percentage of total loans	3.50%	[1]	4.10%	[1]
West North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	223.2	[1]	205.2	[1]
Loans by region percentage of total loans	6.80%	[1]	7.10%	[1]
East South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 78.4	[1]	$ 79.2	[1]
Loans by region percentage of total loans	2.40%	[1]	2.80%	[1]

[1] Balances do not include allowance for mortgage loan credit losses.