Investments - Mortgages by Year of Origination (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Investment [Line Items]
Total commercial mortgage loans	$ 3,062.0	[1]	$ 2,874.0	[1]
Year of Origination 2013
Investment [Line Items]
Total commercial mortgage loans	218.7	[1]	0	[1]
Year of Origination 2012
Investment [Line Items]
Total commercial mortgage loans	934.3	[1]	939.0	[1]
Year of Origination 2011
Investment [Line Items]
Total commercial mortgage loans	826.1	[1]	836.9	[1]
Year of Origination 2010
Investment [Line Items]
Total commercial mortgage loans	123.5	[1]	124.0	[1]
Year of Origination 2009
Investment [Line Items]
Total commercial mortgage loans	72.8	[1]	73.0	[1]
Year of Origination 2008
Investment [Line Items]
Total commercial mortgage loans	117.9	[1]	119.0	[1]
Year of Origination 2007 and Prior
Investment [Line Items]
Total commercial mortgage loans	$ 768.7	[1]	$ 782.1	[1]

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