Investments - Loans by Property Type (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Investment Holdings [Line Items]
Total Commercial mortgage loans	$ 9,953.1	[1]	$ 9,316.0	[1]
Loans by property type percentage of total loans	100.00%		100.00%
Industrial
Investment Holdings [Line Items]
Total Commercial mortgage loans	2,769.3	[1]	2,848.0	[1]
Loans by property type percentage of total loans	27.80%		30.60%
Retail
Investment Holdings [Line Items]
Total Commercial mortgage loans	3,190.4	[1]	2,936.9	[1]
Loans by property type percentage of total loans	32.10%		31.50%
Office
Investment Holdings [Line Items]
Total Commercial mortgage loans	1,226.3	[1]	1,242.2	[1]
Loans by property type percentage of total loans	12.30%		13.30%
Apartments
Investment Holdings [Line Items]
Total Commercial mortgage loans	1,582.2	[1]	1,296.1	[1]
Loans by property type percentage of total loans	15.90%		13.90%
Hotel/Motel
Investment Holdings [Line Items]
Total Commercial mortgage loans	390.3	[1]	430.6	[1]
Loans by property type percentage of total loans	3.90%		4.60%
Mixed Use
Investment Holdings [Line Items]
Total Commercial mortgage loans	347.8	[1]	184.1	[1]
Loans by property type percentage of total loans	3.50%		2.00%
Other
Investment Holdings [Line Items]
Total Commercial mortgage loans	$ 446.8	[1]	$ 378.1	[1]
Loans by property type percentage of total loans	4.50%		4.10%

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