Investments - OTTI (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) securities	Jun. 30, 2015 USD ($) securities	Jun. 30, 2016 USD ($) securities	Jun. 30, 2015 USD ($) securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 1.1	$ 2.4	$ 8.4	$ 4.3
No. of Securities, Credit Impairments | securities	15	19	23	26
Write-downs related to credit impairments	$ 0.6	$ 2.2	$ 1.0	$ 3.1
Impairment, Intent impairments	$ 0.5	$ 0.2	$ 7.4	$ 1.2
No. of Securities, Intent Impairments | securities	1	2	3	4
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 0.0	$ 0.0	$ 0.1	$ 0.0
No. of Securities, Credit Impairments | securities	0	0	1	0
Foreign corporate public securities and foreign governments
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 0.4	$ 1.8	$ 7.3	$ 2.3
No. of Securities, Credit Impairments | securities	1	2	2	2
Impairment, Intent impairments	$ 0.0	$ 0.2	$ 6.9	$ 0.7
No. of Securities, Intent Impairments | securities	0	1	1	1
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 0.1	$ 0.5	$ 0.1	$ 0.5
No. of Securities, Credit Impairments | securities	1	1	1	1
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 0.6	$ 0.1	$ 0.9	$ 1.5
No. of Securities, Credit Impairments | securities	13	16	19	23
Impairment, Intent impairments	$ 0.5	$ 0.0	$ 0.5	$ 0.5
No. of Securities, Intent Impairments | securities	1	1	2	3