Investments - Loans by Property Type (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Investment Holdings [Line Items]
Total Commercial mortgage loans	$ 9,261.5	[1]	$ 9,316.0	[1]
Loans by property type percentage of total loans	100.00%	[1]	100.00%	[1]
Industrial
Investment Holdings [Line Items]
Total Commercial mortgage loans	2,818.6	[1]	2,848.0	[1]
Loans by property type percentage of total loans	30.40%	[1]	30.60%	[1]
Retail
Investment Holdings [Line Items]
Total Commercial mortgage loans	2,951.0	[1]	2,936.9	[1]
Loans by property type percentage of total loans	31.90%	[1]	31.50%	[1]
Office
Investment Holdings [Line Items]
Total Commercial mortgage loans	1,089.0	[1]	1,242.2	[1]
Loans by property type percentage of total loans	11.70%	[1]	13.30%	[1]
Apartments
Investment Holdings [Line Items]
Total Commercial mortgage loans	1,348.7	[1]	1,296.1	[1]
Loans by property type percentage of total loans	14.60%	[1]	13.90%	[1]
Hotel/Motel
Investment Holdings [Line Items]
Total Commercial mortgage loans	382.1	[1]	430.6	[1]
Loans by property type percentage of total loans	4.10%	[1]	4.60%	[1]
Mixed Use
Investment Holdings [Line Items]
Total Commercial mortgage loans	305.8	[1]	184.1	[1]
Loans by property type percentage of total loans	3.30%	[1]	2.00%	[1]
Other
Investment Holdings [Line Items]
Total Commercial mortgage loans	$ 366.3	[1]	$ 378.1	[1]
Loans by property type percentage of total loans	4.00%	[1]	4.10%	[1]

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