Investments - Risk Exposure on Mortgage Backed Securities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Concentration Risk [Line Items]
Fair value of investments	$ 21,378.3	$ 18,791.4
Gross unrealized losses	52.6	152.2
Subprime mortgage-backed securities
Concentration Risk [Line Items]
Fair value of investments	61.2	59.1
Gross unrealized losses	6.2	21.7
Percent of total fixed maturities	0.30%	0.30%
Credit exposure	100.00%	100.00%
Subprime mortgage-backed securities | Vintage Year 2007
Concentration Risk [Line Items]
Credit exposure	8.00%	9.10%
Subprime mortgage-backed securities | Vintage Year 2006
Concentration Risk [Line Items]
Credit exposure	6.00%	4.50%
Subprime mortgage-backed securities | Vintage Year 2005 and Prior
Concentration Risk [Line Items]
Credit exposure	86.00%	86.40%
Subprime mortgage-backed securities | NAIC Designation
Concentration Risk [Line Items]
Credit exposure	100.00%	100.00%
Subprime mortgage-backed securities | NAIC Designation | NAIC Designation of 1
Concentration Risk [Line Items]
Credit exposure	67.80%	75.80%
Subprime mortgage-backed securities | NAIC Designation | NAIC Designation of 2
Concentration Risk [Line Items]
Credit exposure	3.20%	5.30%
Subprime mortgage-backed securities | NAIC Designation | NAIC Designation of 3
Concentration Risk [Line Items]
Credit exposure	19.60%	9.30%
Subprime mortgage-backed securities | NAIC Designation | NAIC Designation of 4
Concentration Risk [Line Items]
Credit exposure	8.70%	9.40%
Subprime mortgage-backed securities | NAIC Designation | NAIC Designation of 5
Concentration Risk [Line Items]
Credit exposure	0.50%	0.00%
Subprime mortgage-backed securities | NAIC Designation | NAIC Designation of 6
Concentration Risk [Line Items]
Credit exposure	0.20%	0.20%
Subprime mortgage-backed securities | Acceptable Rating Organizations
Concentration Risk [Line Items]
Credit exposure	100.00%	100.00%
Subprime mortgage-backed securities | Acceptable Rating Organizations | AAA Rating
Concentration Risk [Line Items]
Credit exposure	3.20%	7.50%
Subprime mortgage-backed securities | Acceptable Rating Organizations | AA Rating
Concentration Risk [Line Items]
Credit exposure	0.00%	0.00%
Subprime mortgage-backed securities | Acceptable Rating Organizations | A Rating
Concentration Risk [Line Items]
Credit exposure	16.20%	13.00%
Subprime mortgage-backed securities | Acceptable Rating Organizations | BBB Rating
Concentration Risk [Line Items]
Credit exposure	21.50%	33.70%
Subprime mortgage-backed securities | Acceptable Rating Organizations | BB and Below Rating
Concentration Risk [Line Items]
Credit exposure	59.10%	45.80%
Alt-A residential mortgage-backed securities
Concentration Risk [Line Items]
Fair value of investments	106.0	111.4
Gross unrealized losses	9.5	19.6
Percent of total fixed maturities	0.50%	0.60%
Credit exposure	100.00%	100.00%
Alt-A residential mortgage-backed securities | Vintage Year 2007
Concentration Risk [Line Items]
Credit exposure	13.80%	12.00%
Alt-A residential mortgage-backed securities | Vintage Year 2006
Concentration Risk [Line Items]
Credit exposure	29.30%	28.30%
Alt-A residential mortgage-backed securities | Vintage Year 2005 and Prior
Concentration Risk [Line Items]
Credit exposure	56.90%	59.70%
Alt-A residential mortgage-backed securities | NAIC Designation
Concentration Risk [Line Items]
Credit exposure	100.00%	100.00%
Alt-A residential mortgage-backed securities | NAIC Designation | NAIC Designation of 1
Concentration Risk [Line Items]
Credit exposure	33.40%	39.90%
Alt-A residential mortgage-backed securities | NAIC Designation | NAIC Designation of 2
Concentration Risk [Line Items]
Credit exposure	12.40%	14.90%
Alt-A residential mortgage-backed securities | NAIC Designation | NAIC Designation of 3
Concentration Risk [Line Items]
Credit exposure	21.00%	14.70%
Alt-A residential mortgage-backed securities | NAIC Designation | NAIC Designation of 4
Concentration Risk [Line Items]
Credit exposure	30.30%	21.10%
Alt-A residential mortgage-backed securities | NAIC Designation | NAIC Designation of 5
Concentration Risk [Line Items]
Credit exposure	2.30%	4.70%
Alt-A residential mortgage-backed securities | NAIC Designation | NAIC Designation of 6
Concentration Risk [Line Items]
Credit exposure	0.60%	4.70%
Alt-A residential mortgage-backed securities | Acceptable Rating Organizations
Concentration Risk [Line Items]
Credit exposure	100.00%	100.00%
Alt-A residential mortgage-backed securities | Acceptable Rating Organizations | AAA Rating
Concentration Risk [Line Items]
Credit exposure	0.20%	0.30%
Alt-A residential mortgage-backed securities | Acceptable Rating Organizations | AA Rating
Concentration Risk [Line Items]
Credit exposure	1.40%	3.10%
Alt-A residential mortgage-backed securities | Acceptable Rating Organizations | A Rating
Concentration Risk [Line Items]
Credit exposure	3.40%	13.10%
Alt-A residential mortgage-backed securities | Acceptable Rating Organizations | BBB Rating
Concentration Risk [Line Items]
Credit exposure	5.60%	4.60%
Alt-A residential mortgage-backed securities | Acceptable Rating Organizations | BB and Below Rating
Concentration Risk [Line Items]
Credit exposure	89.40%	78.90%
Commercial mortgage-backed securities
Concentration Risk [Line Items]
Fair value of investments	839.1	911.3
Gross unrealized losses	0.2	5.8
Credit exposure	100.00%	100.00%
Commercial mortgage-backed securities | Vintage Year 2007
Concentration Risk [Line Items]
Credit exposure	28.70%	23.40%
Commercial mortgage-backed securities | Vintage Year 2006
Concentration Risk [Line Items]
Credit exposure	20.40%	18.20%
Commercial mortgage-backed securities | Vintage Year 2005 and Prior
Concentration Risk [Line Items]
Credit exposure	50.90%	58.40%
Commercial mortgage-backed securities | NAIC Designation
Concentration Risk [Line Items]
Credit exposure	100.00%	100.00%
Commercial mortgage-backed securities | NAIC Designation | NAIC Designation of 1
Concentration Risk [Line Items]
Credit exposure	99.90%	97.40%
Commercial mortgage-backed securities | NAIC Designation | NAIC Designation of 2
Concentration Risk [Line Items]
Credit exposure	0.00%	0.90%
Commercial mortgage-backed securities | NAIC Designation | NAIC Designation of 3
Concentration Risk [Line Items]
Credit exposure	0.10%	0.70%
Commercial mortgage-backed securities | NAIC Designation | NAIC Designation of 4
Concentration Risk [Line Items]
Credit exposure	0.00%	1.00%
Commercial mortgage-backed securities | NAIC Designation | NAIC Designation of 5
Concentration Risk [Line Items]
Credit exposure	0.00%	0.00%
Commercial mortgage-backed securities | NAIC Designation | NAIC Designation of 6
Concentration Risk [Line Items]
Credit exposure	0.00%	0.00%
Commercial mortgage-backed securities | Acceptable Rating Organizations
Concentration Risk [Line Items]
Credit exposure	100.00%	100.00%
Commercial mortgage-backed securities | Acceptable Rating Organizations | AAA Rating
Concentration Risk [Line Items]
Credit exposure	54.10%	63.70%
Commercial mortgage-backed securities | Acceptable Rating Organizations | AA Rating
Concentration Risk [Line Items]
Credit exposure	17.10%	1.40%
Commercial mortgage-backed securities | Acceptable Rating Organizations | A Rating
Concentration Risk [Line Items]
Credit exposure	8.40%	21.10%
Commercial mortgage-backed securities | Acceptable Rating Organizations | BBB Rating
Concentration Risk [Line Items]
Credit exposure	5.30%	4.00%
Commercial mortgage-backed securities | Acceptable Rating Organizations | BB and Below Rating
Concentration Risk [Line Items]
Credit exposure	15.10%	9.80%
Other asset-backed securities
Concentration Risk [Line Items]
Fair value of investments	435.6	381.0
Gross unrealized losses	$ 0.6	$ 0.7
Credit exposure	100.00%	100.00%
Other asset-backed securities | Vintage Year 2012
Concentration Risk [Line Items]
Credit exposure	21.40%
Other asset-backed securities | Vintage Year 2011
Concentration Risk [Line Items]
Credit exposure	12.20%	14.30%
Other asset-backed securities | Vintage Year 2010
Concentration Risk [Line Items]
Credit exposure	5.70%	7.30%
Other asset-backed securities | Vintage Year 2009
Concentration Risk [Line Items]
Credit exposure	0.30%	0.40%
Other asset-backed securities | Vintage Year 2008
Concentration Risk [Line Items]
Credit exposure	9.50%	11.70%
Other asset-backed securities | Vintage Year 2007
Concentration Risk [Line Items]
Credit exposure	22.90%	30.30%
Other asset-backed securities | Vintage Year 2006
Concentration Risk [Line Items]
Credit exposure	6.10%	6.80%
Other asset-backed securities | Vintage Year 2005 and Prior
Concentration Risk [Line Items]
Credit exposure	21.90%	29.20%
Other asset-backed securities | NAIC Designation
Concentration Risk [Line Items]
Credit exposure	100.00%	100.00%
Other asset-backed securities | NAIC Designation | NAIC Designation of 1
Concentration Risk [Line Items]
Credit exposure	98.30%	95.00%
Other asset-backed securities | NAIC Designation | NAIC Designation of 2
Concentration Risk [Line Items]
Credit exposure	1.60%	4.70%
Other asset-backed securities | NAIC Designation | NAIC Designation of 3
Concentration Risk [Line Items]
Credit exposure	0.10%	0.00%
Other asset-backed securities | NAIC Designation | NAIC Designation of 4
Concentration Risk [Line Items]
Credit exposure	0.00%	0.30%
Other asset-backed securities | NAIC Designation | NAIC Designation of 5
Concentration Risk [Line Items]
Credit exposure	0.00%	0.00%
Other asset-backed securities | NAIC Designation | NAIC Designation of 6
Concentration Risk [Line Items]
Credit exposure	0.00%	0.00%
Other asset-backed securities | Acceptable Rating Organizations
Concentration Risk [Line Items]
Credit exposure	100.00%	100.00%
Other asset-backed securities | Acceptable Rating Organizations | AAA Rating
Concentration Risk [Line Items]
Credit exposure	88.40%	82.70%
Other asset-backed securities | Acceptable Rating Organizations | AA Rating
Concentration Risk [Line Items]
Credit exposure	1.90%	1.20%
Other asset-backed securities | Acceptable Rating Organizations | A Rating
Concentration Risk [Line Items]
Credit exposure	8.00%	8.40%
Other asset-backed securities | Acceptable Rating Organizations | BBB Rating
Concentration Risk [Line Items]
Credit exposure	1.60%	7.40%
Other asset-backed securities | Acceptable Rating Organizations | BB and Below Rating
Concentration Risk [Line Items]
Credit exposure	0.10%	0.30%
Credit Card Receivables
Concentration Risk [Line Items]
Percent of total fixed maturities	47.00%	49.30%
Nonconsolidated Collateralized Loan Obligations
Concentration Risk [Line Items]
Percent of total fixed maturities	5.60%	5.50%
Automobile Receivables
Concentration Risk [Line Items]
Percent of total fixed maturities	26.90%	17.20%