Significant Transactions (Details) $ / shares in Units, ₪ in Millions, $ in Millions			1 Months Ended			2 Months Ended				3 Months Ended
	Feb. 01, 2016 USD ($)	Nov. 06, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jan. 31, 2014 USD ($)	Oct. 31, 2011 USD ($)	Feb. 23, 2015 ILS (₪)	Feb. 23, 2015 USD ($)	Feb. 19, 2014 ILS (₪)	Feb. 19, 2014 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($) $ / shares	Mar. 31, 2016 USD ($) $ / shares	Jun. 30, 2015 ILS (₪)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Sep. 01, 2015 USD ($)	Oct. 10, 2014 USD ($)	Mar. 14, 2014 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Other Investments											$ (10.9)				$ (6.5)
Investments											3,639.7	$ 3,639.7						$ 4,280.7
Sales of common equity securities											767.5				59.9
Loss and loss adjustment expenses											161.1				167.9
Payments to Acquire Businesses and Interest in Affiliates											8.1				2.4
Payment of contingent consideration											(7.8)				0.0
Assets											10,391.8	10,391.8						10,282.6
Goodwill											187.9	187.9			169.2	$ 168.9		187.9
Intangible Assets, Net (Excluding Goodwill)											183.6	183.6			178.2	182.3		187.8
Goodwill and intangible assets											371.5	371.5						375.7
Liabilities											6,154.5	6,154.5						5,914.6
Net Income (Loss) Attributable to Noncontrolling Interest											(6.3)				7.3
Cancerinsurance.com [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates		$ 3.1
Cash		1.1
Business Combination, Contingent Consideration, Liability																		6.8
Liabilities		$ 2.0
Tranzact [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates					$ 177.7
Enterprise Value of Subsidiary																				$ 281.2
Proceeds from Contributions from Affiliates																$ 44.2
Business Combination, Contingent Consideration, Liability																				$ 7.4
Ownership interest (as a percent)																				63.20%
Assets																				$ 332.8
Tangible Assets																				41.4
Goodwill																				145.1
Intangible Assets, Net (Excluding Goodwill)																				146.3
Liabilities																				$ 108.7
TruBridge [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates			$ 31.0
Ownership interest (as a percent)																			100.00%
Assets																			$ 54.5
Goodwill																			18.7
Intangible Assets, Net (Excluding Goodwill)																			28.1
Liabilities																			$ 4.3
Star & Shield Insurance Exchange
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase of Surplus Notes											21.0	21.0
Assets											13.3	13.3						14.2
Liabilities											29.4	29.4						30.3
MediaAlpha [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates																	$ 28.1
Percentage of gross profit																					62.50%
Payment of contingent consideration											$ 7.8
Assets																					$ 70.1
Goodwill																					18.3
Goodwill and intangible assets																					38.5
Liabilities																					$ 10.0
WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of Business in the Insurance Market											85.00%
Goodwill									$ 5.5
Liabilities									$ 0.7
Star & Shield LLC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates				$ 1.8
Lowfares.com [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates												$ 3.9
OneBeacon [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of Quota Share Reinsurance Agreement											50.00%
Common Class A | MediaAlpha [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)																					60.00%
Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)									54.00%		96.10%	96.10%
Non-controlling interest | Star & Shield Insurance Exchange
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Income (Loss) Attributable to Noncontrolling Interest											$ (0.1)				(1.1)
Israel, New Shekels | Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Additional Interest in Subsidiaries | ₪													₪ 91.4
Israel, New Shekels | Preferred Stock [Member] | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪								₪ 31.5
Israel, New Shekels | Common Stock | Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪						₪ 35.9
Israel, New Shekels | Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪								₪ 14.4
United States of America, Dollars | Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Additional Interest in Subsidiaries														$ 23.8
United States of America, Dollars | Preferred Stock [Member] | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates									$ 9.0
United States of America, Dollars | Common Stock | Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates							$ 8.9
United States of America, Dollars | Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates									4.1
Lowfares.com [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates									3.9
Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets							5.5
Goodwill							0.3
Intangible Assets, Net (Excluding Goodwill)							2.8
Liabilities							$ 1.2
WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets									13.4
Intangible Assets, Net (Excluding Goodwill)									$ 2.9
Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Divestiture of Interest in Subsidiaries and Affiliates										$ 2,600.0
Payments to Acquire Other Investments										161.8
Investments											$ 161.8	$ 161.8						686.2
Essentia [Member] | OneBeacon [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (Loss) from Adjustments to Prior Period Extraordinary Items															$ 3.7
Crop insurance [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds related from termination of agreement																		3.0
Percentage of Quota Share Reinsurance Agreement											100.00%
Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of common equity securities	$ 658.0
Expected [Member] | Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Dividends Payable, Amount Per Share | $ / shares											$ 32.00	$ 32.00
Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of common equity securities											$ 658.0
Symetra | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investments																		$ 528.6
UNITED STATES
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Industry Loss Warranty Limit on Contract											45.0
Industry Loss Trigger on Industry Loss Warranty											15,000.0
JAPAN
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Industry Loss Warranty Limit on Contract											25.0
Industry Loss Trigger on Industry Loss Warranty											12,500.0
Combination One [Member] | UNITED STATES
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Industry Loss Warranty Limit on Contract											75.0
Industry Loss Trigger on Industry Loss Warranty											40,000.0
Combination Two [Member] | UNITED STATES
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Industry Loss Warranty Limit on Contract											22.5
Industry Loss Trigger on Industry Loss Warranty											$ 50,000.0
Subsequent Event [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Divestiture of Interest in Subsidiaries and Affiliates										2,600.0
Payments to Acquire Other Investments										161.8
Subsequent Event [Member] | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Divestiture of Interest in Subsidiaries and Affiliates										2,592.0
Payments to Acquire Other Investments										$ 161.8