Investments - OTTI (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) securities	Mar. 31, 2016 USD ($) securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 0.1	$ 7.3
No. of Securities, Credit Impairments | securities	8	16
Write-downs related to credit impairments	$ 0.1	$ 0.4
Impairment, Intent impairments	$ 0.0	$ 6.9
No. of Securities, Intent Impairments | securities	1	2
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 0.0	$ 0.1
No. of Securities, Credit Impairments | securities	0	1
Impairment, Intent impairments	$ 0.0	$ 0.0
No. of Securities, Intent Impairments | securities	0	0
Foreign corporate public securities and foreign governments
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 0.0	$ 6.9
No. of Securities, Credit Impairments | securities	0	1
Impairment, Intent impairments	$ 0.0	$ 6.9
No. of Securities, Intent Impairments | securities	0	1
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment, Credit impairments	$ 0.1	$ 0.3
No. of Securities, Credit Impairments | securities	8	14
Impairment, Intent impairments	$ 0.0	$ 0.0
No. of Securities, Intent Impairments | securities	1	1