Investments - Loans by Property Type (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 3,284.2	[1]	$ 2,874.0	[1]
Loans by property type percentage of total loans	100.00%	[1]	100.00%	[1]
Industrial
Investment Holdings [Line Items]
Total commercial mortgage loans	1,028.6	[1]	1,035.2	[1]
Loans by property type percentage of total loans	31.30%	[1]	36.00%	[1]
Retail
Investment Holdings [Line Items]
Total commercial mortgage loans	1,041.2	[1]	824.0	[1]
Loans by property type percentage of total loans	31.60%	[1]	28.70%	[1]
Office
Investment Holdings [Line Items]
Total commercial mortgage loans	448.9	[1]	427.0	[1]
Loans by property type percentage of total loans	13.70%	[1]	14.80%	[1]
Apartments
Investment Holdings [Line Items]
Total commercial mortgage loans	372.8	[1]	298.7	[1]
Loans by property type percentage of total loans	11.40%	[1]	10.40%	[1]
Hotel/Motel
Investment Holdings [Line Items]
Total commercial mortgage loans	139.7	[1]	92.1	[1]
Loans by property type percentage of total loans	4.30%	[1]	3.20%	[1]
Mixed Use
Investment Holdings [Line Items]
Total commercial mortgage loans	70.2	[1]	34.2	[1]
Loans by property type percentage of total loans	2.10%	[1]	1.20%	[1]
Other
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 182.8	[1]	$ 162.8	[1]
Loans by property type percentage of total loans	5.60%	[1]	5.70%	[1]

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