Goodwill and Intangible Assets - Finite-lived Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ 157.7	$ 109.5	$ 94.0
Policies in force [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	256.2	256.2	256.2
Accumulated Amortization	91.5	64.1	55.0
Net Carrying Amount	164.7	192.1	201.2
Agency relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	159.2	159.2	159.2
Accumulated Amortization	28.4	19.9	17.1
Net Carrying Amount	130.8	139.3	142.1
Software rights [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	79.1	79.1	79.1
Accumulated Amortization	26.8	18.8	16.1
Net Carrying Amount	52.3	60.3	63.0
Trade name [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	34.8	34.8	34.8
Accumulated Amortization	10.4	6.1	5.2
Net Carrying Amount	24.4	28.7	29.6
Finite-Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	529.3	529.3	529.3
Accumulated Amortization	157.1	108.9	93.4
Net Carrying Amount	$ 372.2	$ 420.4	$ 435.9