Investments - OTTI (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) securities	Sep. 30, 2016 USD ($) securities	Sep. 30, 2017 USD ($) securities	Sep. 30, 2016 USD ($) securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment	$ 0.2	$ 4.3	$ 0.6	$ 12.7
No. of Securities | securities	6	9	16	26
Write-downs related to credit impairments	$ 0.2	$ 0.6	$ 0.3	$ 1.6
Impairment, Intent impairments	$ 0.0	$ 3.7	$ 0.3	$ 11.1
No. of Securities, Intent Impairments | securities	1	1	5	4
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment	$ 0.0	$ 3.7	$ 0.1	$ 3.8
No. of Securities | securities	0	1	1	2
Impairment, Intent impairments	$ 0.0	$ 3.7	$ 0.1	$ 3.7
No. of Securities, Intent Impairments | securities	0	1	1	1
Foreign corporate public securities and foreign governments
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment	$ 0.0	$ 0.0	$ 0.0	$ 7.3
No. of Securities | securities	0	0	0	2
Impairment, Intent impairments	$ 0.0	$ 0.0	$ 0.0	$ 6.9
No. of Securities, Intent Impairments | securities	0	0	0	1
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment	$ 0.1	$ 0.6	$ 0.1	$ 0.7
No. of Securities | securities	1	1	1	2
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment	$ 0.1	$ 0.0	$ 0.4	$ 0.9
No. of Securities | securities	5	7	13	20
Impairment, Intent impairments	$ 0.0	$ 0.0	$ 0.2	$ 0.5
No. of Securities, Intent Impairments | securities	1	0	3	2
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment	$ 0.0	$ 0.0	$ 0.0	$ 0.0
No. of Securities | securities	0	0	1	0
Impairment, Intent impairments	$ 0.0	$ 0.0	$ 0.0	$ 0.0
No. of Securities, Intent Impairments | securities	0	0	1	0