Goodwill and Intangible Assets - Finite-lived Intangible Assets (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ 211.7	$ 175.7	$ 140.5
Policies in force [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	256.2	256.2	256.2
Accumulated Amortization	119.0	100.7	82.4
Net Carrying Amount	137.2	155.5	173.8
Agency relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	159.2	159.2	159.2
Accumulated Amortization	37.0	31.3	25.6
Net Carrying Amount	122.2	127.9	133.6
Software rights [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	79.1	79.1	79.1
Accumulated Amortization	34.7	29.4	24.1
Net Carrying Amount	44.4	49.7	55.0
Trade name [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	34.8	34.8	34.8
Accumulated Amortization	20.4	13.7	7.8
Net Carrying Amount	14.4	21.1	27.0
Finite-Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	529.3	529.3	529.3
Accumulated Amortization	211.1	175.1	139.9
Net Carrying Amount	$ 318.2	$ 354.2	$ 389.4