Investments - Loans by U.S. Region (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 9,261.5	[1]	$ 9,316.0	[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	2,369.2	[1]	2,281.8	[1]
Loans by region percentage of total loans	25.60%	[1]	24.50%	[1]
South Atlantic
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	1,964.6	[1]	1,936.0	[1]
Loans by region percentage of total loans	21.20%	[1]	20.80%	[1]
Middle Atlantic
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	1,113.2	[1]	1,112.0	[1]
Loans by region percentage of total loans	12.00%	[1]	11.90%	[1]
West South Central
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	1,112.7	[1]	1,122.3	[1]
Loans by region percentage of total loans	12.00%	[1]	12.00%	[1]
East North Central
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	1,055.6	[1]	1,037.5
Loans by region percentage of total loans	11.40%	[1]	11.10%	[1]
Mountain
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	771.7	[1]	790.4	[1]
Loans by region percentage of total loans	8.30%	[1]	8.50%	[1]
West North Central
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	525.6	[1]	517.2	[1]
Loans by region percentage of total loans	5.70%	[1]	5.60%	[1]
East South Central
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	198.7	[1]	200.7	[1]
Loans by region percentage of total loans	2.20%	[1]	2.20%	[1]
New England
Open Option Contracts Written [Line Items]
Total Commercial mortgage loans	$ 150.2	[1]	$ 318.1
Loans by region percentage of total loans	1.60%	[1]	3.40%	[1]

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