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	$ 9,019.6	[1]	$ 8,666.2	[1]
Loans by property type percentage of total loans	100.00%	[1]	100.00%	[1]
Industrial
Investment Holdings [Line Items]
Total Commercial mortgage loans	2,998.0	[1]	3,361.5	[1]
Loans by property type percentage of total loans	33.30%	[1]	38.80%	[1]
Retail
Investment Holdings [Line Items]
Total Commercial mortgage loans	2,795.9	[1]	2,350.2	[1]
Loans by property type percentage of total loans	31.00%	[1]	27.10%	[1]
Office
Investment Holdings [Line Items]
Total Commercial mortgage loans	1,243.3	[1]	1,284.7	[1]
Loans by property type percentage of total loans	13.80%	[1]	14.80%	[1]
Apartments
Investment Holdings [Line Items]
Total Commercial mortgage loans	1,048.6	[1]	952.1	[1]
Loans by property type percentage of total loans	11.60%	[1]	11.00%	[1]
Hotel/Motel
Investment Holdings [Line Items]
Total Commercial mortgage loans	364.1	[1]	280.6	[1]
Loans by property type percentage of total loans	4.00%	[1]	3.20%	[1]
Mixed Use
Investment Holdings [Line Items]
Total Commercial mortgage loans	187.4	[1]	74.0	[1]
Loans by property type percentage of total loans	2.10%	[1]	0.90%	[1]
Other
Investment Holdings [Line Items]
Total Commercial mortgage loans	$ 382.3	[1]	$ 363.1	[1]
Loans by property type percentage of total loans	4.20%	[1]	4.20%	[1]

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