Investments (excluding Consolidated Investment Entities) - OTTI (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($) security	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($) security
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	$ 1.0	$ 1.0	$ 15.0	$ 3.0
No. of Securities | security	18	20	26	41
Write-downs related to credit impairments	$ 0.0	$ 0.0	$ 15.0	$ 2.0
Impairment, Intent Impairments	$ 0.0	$ 1.0	$ 0.0	$ 1.0
No. of Securities, Intent Impairments | security	2	3	5	10
State, municipalities and political subdivisions
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	$ 0.0	$ 0.0	$ 0.0	$ 1.0
No. of Securities | security	0	3	0	3
U.S. corporate public securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	$ 0.0	$ 0.0	$ 0.0	$ 0.0
No. of Securities | security	0	1	0	1
Impairment, Intent Impairments	$ 0.0	$ 0.0	$ 0.0	$ 0.0
No. of Securities, Intent Impairments | security	0	1	0	1
Foreign corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	$ 0.0	$ 0.0	$ 14.0	$ 0.0
No. of Securities | security	0	0	1	0
Residential mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	$ 1.0	$ 1.0	$ 1.0	$ 1.0
No. of Securities | security	18	15	25	34
Impairment, Intent Impairments	$ 0.0	$ 1.0	$ 0.0	$ 0.0
No. of Securities, Intent Impairments | security	2	1	5	6
Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Impairment	$ 0.0	$ 0.0	$ 0.0	$ 1.0
No. of Securities | security	0	1	0	3
Impairment, Intent Impairments	$ 0.0	$ 0.0	$ 0.0	$ 1.0
No. of Securities, Intent Impairments | security	0	1	0	3