Subsequent Events (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Jul. 27, 2015
Subsequent Event [Line Items]
Other Underwriting Expense		$ 80.7	$ 81.9	$ 162.2	$ 163.3
UNITED STATES | Sale of Sirius [Member]
Subsequent Event [Line Items]
Stop Loss Limitation				45.0
Industry Loss Trigger on Industry Loss Warranty				15,000.0
UNITED STATES | Combination One | Sale of Sirius [Member]
Subsequent Event [Line Items]
Stop Loss Limitation				75.0
Industry Loss Trigger on Industry Loss Warranty				40,000.0
UNITED STATES | Combination Two | Sale of Sirius [Member]
Subsequent Event [Line Items]
Stop Loss Limitation				22.5
Industry Loss Trigger on Industry Loss Warranty				50,000.0
JAPAN | Sale of Sirius [Member]
Subsequent Event [Line Items]
Stop Loss Limitation				25.0
Industry Loss Trigger on Industry Loss Warranty				12,500.0
Sirius Group | Sale of Sirius [Member]
Subsequent Event [Line Items]
Percentage of common shareholder's equity							127.30%
Additional cash that will be paid as part of the purchase price							$ 10.0
Payments for (Proceeds from) Businesses and Interest in Affiliates				2,200.0
OneBeacon | Crop-hail insurance [Member]
Subsequent Event [Line Items]
Years of Agreement	5 years
Proceeds from Divestiture of Businesses and Interests in Affiliates	$ 3.0
Percentage of Quota Share Reinsurance Agreement	100.00%
Premiums Written, Net				26.6		$ 35.1
Underwriting Income (Loss)				(1.3)		(3.5)
Other Underwriting Expense				$ 1.5		$ 1.0