Investments - OTTI (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014 security	Sep. 30, 2013 security	Sep. 30, 2014 security	Sep. 30, 2013 security
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	$ 19.4	$ 4.4	$ 25.2	$ 22.6
No. of Securities	115	48	158	118
U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	0.5	0	0.5	0
No. of Securities	1	0	1	0
U.S. corporate securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	14.4	0	14.8	0
No. of Securities	41	0	41	0
Foreign
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	1.3	0	3.0	0
No. of Securities	4	0	9	0
Residential mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	2.5	1.1	4.9	9.9
No. of Securities	47	39	81	105
Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	0	0.3	0.2	0.4
No. of Securities	5	1	7	3
Other asset-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	0.7	1.1	0.8	8.6
No. of Securities	17	5	17	7
Equity securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	0	1.2	1.0	3.0
No. of Securities	0	2	2	2
Other assets
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	0	0.7	0	0.7
No. of Securities	0	1	0	1
Intent related impairment
Available-for-sale Securities, Including Securities Pledged [Line Items]
No. of Securities	81	7	95	11
Write-down related to intent impairments	17.1	0.8	19.3	8.2
Intent related impairment | U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
No. of Securities	1	0	1	0
Write-down related to intent impairments	0.5	0	0.5	0
Intent related impairment | U.S. corporate securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
No. of Securities	41	0	41	0
Write-down related to intent impairments	14.4	0	14.4	0
Intent related impairment | Foreign
Available-for-sale Securities, Including Securities Pledged [Line Items]
No. of Securities	4	0	9	0
Write-down related to intent impairments	1.3	0	3.0	0
Intent related impairment | Residential mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
No. of Securities	16	6	23	6
Write-down related to intent impairments	0.7	0.5	1.0	0.5
Intent related impairment | Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
No. of Securities	5	1	7	3
Write-down related to intent impairments	0	0.3	0.2	0.4
Intent related impairment | Other asset-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
No. of Securities	14	0	14	2
Write-down related to intent impairments	0.2	0	0.2	7.3
Intent related impairment | Equity securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
No. of Securities	0	0	0	0
Write-down related to intent impairments	0	0	0	0
Credit related impairment
Available-for-sale Securities, Including Securities Pledged [Line Items]
Write-downs related to credit impairments	$ 2.3	$ 3.6	$ 5.9	$ 14.4