Investments - Loans by U.S. Region (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 2,874.0	[1]	$ 2,374.8	[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	564.1	[1]	514.7	[1]
Loans by region percentage of total loans	19.60%	[1]	21.70%	[1]
South Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	561.0	[1]	412.0	[1]
Loans by region percentage of total loans	19.50%	[1]	17.30%	[1]
Middle Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	332.7	[1]	325.9	[1]
Loans by region percentage of total loans	11.60%	[1]	13.70%	[1]
East North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	337.8	[1]	285.6	[1]
Loans by region percentage of total loans	11.80%	[1]	12.00%	[1]
West South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	460.4	[1]	358.4	[1]
Loans by region percentage of total loans	16.00%	[1]	15.10%	[1]
Mountain
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	214.5	[1]	191.2	[1]
Loans by region percentage of total loans	7.50%	[1]	8.00%	[1]
New England
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	119.1	[1]	94.2	[1]
Loans by region percentage of total loans	4.10%	[1]	4.00%	[1]
West North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	205.2	[1]	98.9	[1]
Loans by region percentage of total loans	7.10%	[1]	4.20%	[1]
East South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 79.2	[1]	$ 93.9	[1]
Loans by region percentage of total loans	2.80%	[1]	4.00%	[1]

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