Investments - OTTI (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) securities	Mar. 31, 2015 USD ($) securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment	$ 7.3	$ 1.9
No. of Securities | securities	16	20
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment	$ 0.1	$ 0.0
No. of Securities | securities	1	0
Foreign corporate public securities and foreign governments
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment	$ 6.9	$ 0.5
No. of Securities | securities	1	1
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Impairment	$ 0.3	$ 1.4
No. of Securities | securities	14	19
Intent related impairment
Available-for-sale Securities Including Securities Pledged [Line Items]
No. of Securities, Intent-related | securities	2	3
Write-down related to intent impairments	$ 6.9	$ 1.0
Intent related impairment | U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
No. of Securities, Intent-related | securities	0	0
Write-down related to intent impairments	$ 0.0	$ 0.0
Intent related impairment | Foreign corporate public securities and foreign governments
Available-for-sale Securities Including Securities Pledged [Line Items]
No. of Securities, Intent-related | securities	1	1
Write-down related to intent impairments	$ 6.9	$ 0.5
Intent related impairment | Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
No. of Securities, Intent-related | securities	1	2
Write-down related to intent impairments	$ 0.0	$ 0.5
Credit related impairment
Available-for-sale Securities Including Securities Pledged [Line Items]
Write-downs related to credit impairments	$ 0.4	$ 0.9