Amount of cash subject to withdrawal restrictions, restricted deposits held as compensating balances, and cash and securities segregated in compliance with regulations (such as cash deposited in a special reserve account for the exclusive benefit of customers).

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Amount of dividends available for payment after computation of statutory restrictions on capital surplus.

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Statutory Financial Information - Additional Details (Detail) £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 GBP (£)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2016 GBP (£)
Fairwind Insurance Company
Statutory Accounting Practices
Statutory Accounting Practices, Permitted Practice, Amount	$ 142.0	$ 213.0
Unum Limited
Statutory Accounting Practices
Statutory Net Income \| £			£ 25.9
Statutory Combined Capital and Surplus \| £						£ 242.8
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments \| £					£ 260.0
Traditional U.S. Life Insurance Subsidiaries
Statutory Accounting Practices
Statutory Net Income	807.4	855.1		$ 653.7
Statutory Combined Capital and Surplus	3,448.3	3,626.9
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments	796.6
Cash and Securities Segregated under Federal and Other Regulations	$ 155.7	$ 194.9