Dividend Restrictions and Statutory Requirements - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Statutory Accounting Practices [Line Items]
Retained earnings	$ 1,580.0	$ 1,400.0
Restricted net assets of consolidated subsidiaries	1,240.0	1,230.0
Life and Annuities
Statutory Accounting Practices [Line Items]
Minimum RBC requirements	80.8	107.6
Maximum dividend payout	$ 0.5	11.9
Bermuda
Statutory Accounting Practices [Line Items]
Statutory capital (percent)	120.00%
Bermuda | Non Life Underwriting
Statutory Accounting Practices [Line Items]
Minimum liquidity ratio (percent) (not less than)	0.75
Maximum percentage reduction in total statutory capital without prior approval of Bermuda regulator (percent) (or more)	15.00%
Minimum solvency requirements	$ 1,260.0	1,190.0
Minimum liquidity requirements	1,180.0	928.5
United Kingdom | Non-life Run-off
Statutory Accounting Practices [Line Items]
Minimum capital requirements	567.4	442.8
Australia | Non-life Run-off
Statutory Accounting Practices [Line Items]
Minimum solvency requirements	$ 29.2	5.7
Subsidiaries insurance exceeded (percent)	99.50%
United States | Non-life Run-off
Statutory Accounting Practices [Line Items]
Minimum levels of risk-based capital	$ 528.3	549.1
Europe
Statutory Accounting Practices [Line Items]
Minimum solvency requirements	20.4	20.0
Europe | Non-life Run-off
Statutory Accounting Practices [Line Items]
Minimum solvency margin I	111.0	119.2
Minimum Swiss solvency test margin	94.8	85.3
Europe | Life and Annuities
Statutory Accounting Practices [Line Items]
Minimum solvency requirements	17.1	$ 13.3
Belgium | Life and Annuities
Statutory Accounting Practices [Line Items]
Minimum solvency requirements	$ 18.8