Segment Information Segments - Additional Information (Detail) £ in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) Integer Rate	Sep. 30, 2015 GBP (£) Integer	Sep. 30, 2015 GBP (£) Rate	Dec. 31, 2014 USD ($)
Business Combination Segment Allocation [Line Items]
Goodwill | $	$ 232.1			$ 198.7
Number of Operating Segments | Integer	3	3
Date of Acquisition of Subsidiary	Sep. 03, 2015	Sep. 03, 2015
Percentage of Voting Interests Acquired	100.00%		100.00%
Name of Acquired Entity	National Dental Plan Limited and associated companies (National Dental)	National Dental Plan Limited and associated companies (National Dental)
Payments to Acquire Businesses	$ 54.3	£ 35.9
Revenue Reported by Acquired Entity for Last Annual Period	24.2	14.7
Total of Acquired Entity's Identifiable Assets Purchased	28.1	18.6
Total of Acquireed Entity's Liabilities Purchased	7.5	£ 5.0
Unum UK
Business Combination Segment Allocation [Line Items]
Goodwill	$ 33.7		£ 22.3