Investments - OTTI (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) securities	Sep. 30, 2015 USD ($) securities	Sep. 30, 2016 USD ($) securities	Sep. 30, 2015 USD ($) securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 4.3	$ 22.2	$ 12.7	$ 26.5
No. of Securities, Credit Impairments | securities	9	16	26	35
Write-downs related to credit impairments	$ 0.6	$ 0.2	$ 1.6	$ 3.3
Impairment, Intent impairments	$ 3.7	$ 22.0	$ 11.1	$ 23.2
No. of Securities, Intent Impairments | securities	1	10	4	12
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 3.7	$ 1.5	$ 3.8	$ 1.5
No. of Securities, Credit Impairments | securities	1	1	2	1
Impairment, Intent impairments	$ 3.7	$ 1.5	$ 3.7	$ 1.5
No. of Securities, Intent Impairments | securities	1	1	1	1
Foreign corporate public securities and foreign governments
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 0.0	$ 20.0	$ 7.3	$ 22.3
No. of Securities, Credit Impairments | securities	0	6	2	7
Impairment, Intent impairments	$ 0.0	$ 19.9	$ 6.9	$ 20.6
No. of Securities, Intent Impairments | securities	0	6	1	6
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 0.6	$ 0.0	$ 0.7	$ 0.5
No. of Securities, Credit Impairments | securities	1	0	2	1
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 0.0	$ 0.6	$ 0.9	$ 2.1
No. of Securities, Credit Impairments | securities	7	8	20	25
Impairment, Intent impairments	$ 0.0	$ 0.6	$ 0.5	$ 1.1
No. of Securities, Intent Impairments | securities	0	3	2	5
Other asset-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 0.0	$ 0.1	$ 0.0	$ 0.1
No. of Securities, Credit Impairments | securities	0	1	0	1