Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) - Performance Shares $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) multiplier shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares
Performance Cycle From 2013 to 2015
Unamortized grant date fair value
Vested (in shares)		5,000
White Mountains
Share-based compensation arrangement by share-based payment award
Multiplier for determining payout, low end of range \| multiplier	0
Multiplier for determining payout, high end of range \| multiplier	2
Performance cycle period	3 years
W T M Incentive Plan \| White Mountains
Target Performance Share activity
Beginning of period	80,353	93,654	111,257
Shares paid or expired	(30,838)	(36,294)	(42,959)
New grants	17,710	22,615	29,195
Assumed forfeitures and cancellations	(16,710)	378	(3,839)
End of period	50,515	80,353	93,654
Unamortized grant date fair value
Beginning of period \| $	$ 42.4	$ 57.7	$ 44.4
Shares paid or expired \| $	(21.9)	(41.0)	(30.8)
Assumed forfeitures and cancellations \| $	(9.3)	0.5	(0.3)
Expense recognized \| $	34.6	25.2	44.4
End of period \| $	45.8	$ 42.4	$ 57.7
Unamortized grant date fair value \| $	$ 20.2
W T M Incentive Plan \| Sirius Group
Target Performance Share activity
Beginning of period	7,315	10,826
End of period	2,195	7,315	10,826
W T M Incentive and Phantom Plan \| Performance Cycle From 2013 to 2015
Unamortized grant date fair value
Performance goal percentage for minimum payout	34.00%
Payout for maximum growth target percentage	76.00%
W T M Incentive and Phantom Plan \| Performance Cycle From 2012 to 2014
Unamortized grant date fair value
Performance goal percentage for minimum payout		140.00%
Payout for maximum growth target percentage		142.00%
W T M Incentive and Phantom Plan \| Performance Cycle from 2011 to 2013
Unamortized grant date fair value
Performance goal percentage for minimum payout			91.00%
Payout for maximum growth target percentage			145.50%

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901