Statutory Financial Information - Additional Details (Detail) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Fairwind Insurance Company USD ($)	Dec. 31, 2013 Fairwind Insurance Company USD ($)	Dec. 31, 2014 Unum Limited GBP (£)	Dec. 31, 2014 United States Traditional Insurance Subsidiaries USD ($)	Dec. 31, 2013 United States Traditional Insurance Subsidiaries USD ($)	Dec. 31, 2012 United States Traditional Insurance Subsidiaries USD ($)
Statutory Accounting Practices
Statutory Accounting Practices, Permitted Practice, Amount	$ 200	$ 176
Statutory Net Income			54.6	623.1	584.5	624.5
Statutory Combined Capital and Surplus			425.2	3,462.8	3,450.5
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments			166.6	604.9
Cash and Securities Segregated under Federal and Other Regulations				$ 279.1	$ 280.5