Investments - Mortgages by Year of Origination (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Investment [Line Items]
Total commercial mortgage loans	$ 2,874.0	[1]	$ 2,374.8	[1]
Year of Origination 2012
Investment [Line Items]
Total commercial mortgage loans	939.0	[1]	0	[1]
Year of Origination 2011
Investment [Line Items]
Total commercial mortgage loans	836.9	[1]	857.9	[1]
Year of Origination 2010
Investment [Line Items]
Total commercial mortgage loans	124.0	[1]	161.9	[1]
Year of Origination 2009
Investment [Line Items]
Total commercial mortgage loans	73.0	[1]	92.6	[1]
Year of Origination 2008
Investment [Line Items]
Total commercial mortgage loans	119.0	[1]	137.2	[1]
Year of Origination 2007
Investment [Line Items]
Total commercial mortgage loans	102.3	[1]	202.1	[1]
Year of Origination 2006 and Prior
Investment [Line Items]
Total commercial mortgage loans	$ 679.8	[1]	$ 923.1	[1]

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