Investments - Mortgages by Year of Origination (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Investment [Line Items]
Total Commercial mortgage loans	$ 9,261.5	[1]	$ 9,316.0	[1]
Year of Origination 2014
Investment [Line Items]
Total Commercial mortgage loans	252.9	[1]	0	[1]
Year of Origination 2013
Investment [Line Items]
Total Commercial mortgage loans	2,187.0	[1]	2,199.8	[1]
Year of Origination 2012
Investment [Line Items]
Total Commercial mortgage loans	1,731.5	[1]	1,743.3	[1]
Year of Origination 2011
Investment [Line Items]
Total Commercial mortgage loans	1,804.9	[1]	1,835.9	[1]
Year of Origination 2010
Investment [Line Items]
Total Commercial mortgage loans	394.4	[1]	409.8	[1]
Year of Origination 2009
Investment [Line Items]
Total Commercial mortgage loans	148.9	[1]	149.5	[1]
Year of Origination 2008
Investment [Line Items]
Total Commercial mortgage loans	$ 2,741.9	[1]	$ 2,977.7	[1]

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