The date when the equity-based award expires as specified in the award agreement, which may be presented in a variety of ways (for example, year, month and year, day, month and year, quarter of a year).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph a
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Employee Share-Based Incentive Compensation Plans (Details 5) (USD $) In Millions, except Share data, unless otherwise specified			1 Months Ended	3 Months Ended		6 Months Ended					1 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Nov. 30, 2006 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Jun. 30, 2011 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Jun. 30, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Jun. 30, 2011 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Jun. 30, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Nov. 16, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options	May 27, 2008 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Mar. 31, 2008 OneBeacon Long-Term Incentive Plan Non-Qualified Options	May 31, 2010 Non-Qualified Options	Jun. 30, 2010 Non-Qualified Options	Jun. 30, 2011 Non-Qualified Options	Jun. 30, 2010 Non-Qualified Options
Share-based compensation arrangement by share-based payment award
Granted (in shares)			1,420,000
Target options outstanding
Opening balance - outstanding Options (in shares)				768,652	916,827	768,652	1,015,610
Forfeitures and cancellations (in shares)				(18,522)	(37,044)	(18,522)	(37,044)
Vested and expired (in shares)							(98,783)
Ending balance - outstanding Options (in shares)				750,130	879,783	750,130	879,783
Expense Amortized
Accrued expense at the beginning of period	$ 153.3	$ 157.4		$ 4.6	$ 4.0	$ 4.5	$ 3.6
Expense recognized				0.2	0.1	0.3	0.5				8.7	9.7	0.1	11.1
Accrued expense at the end of the period	$ 153.3	$ 157.4		$ 4.8	$ 4.1	$ 4.8	$ 4.1
Options vesting period (in years)						Third, fourth and fifth anniversaries
Expiration period of award (in years)							5.5
Assumptions of stock-based compensation awards
Expected volatility (as a percent)						30.00%
Risk-free rate (as a percent)						4.60%
Forfeiture (as a percent)						5.00%
Expected dividend yield (as a percent)						3.40%
Expected life (in years)						5.5
Initial exercise price (in dollars per share)						$ 30.00
Amended exercise price of option (in dollars per share)								$ 25.47	$ 27.97
OneBeacon special dividend (in dollars per share)								$ 2.50		$ 2.03