Significant Transactions (Details) $ / shares in Units, ₪ in Millions, $ in Millions			1 Months Ended	2 Months Ended				3 Months Ended					6 Months Ended	9 Months Ended
	Apr. 18, 2016 USD ($)	Feb. 01, 2016 USD ($) $ / shares	Jan. 31, 2014 USD ($)	Feb. 23, 2015 ILS (₪)	Feb. 23, 2015 USD ($)	Feb. 19, 2014 ILS (₪)	Feb. 19, 2014 USD ($)	Sep. 30, 2016 ILS (₪)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2016 ILS (₪)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Aug. 04, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2014 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Repayments of Debt															$ 401.8	$ 84.5
Payments to Acquire Other Investments															37.3	30.5
Investments									$ 5,213.5						5,213.5			$ 4,271.1
Sales of common equity securities															961.1	370.6
Payments to Acquire Businesses and Interest in Affiliates															13.7	2.4
Other Income									34.8			$ 38.3			110.8	114.4
Assets									6,683.3						6,683.3			10,282.6
Goodwill										$ 24.1			$ 24.1					24.1	$ 24.1	$ 23.8
Intangible Assets, Net (Excluding Goodwill)										29.2			$ 29.2					31.3	$ 36.4	$ 38.7
Liabilities									2,881.9						2,881.9			5,914.6
Net Income (Loss) Attributable to Noncontrolling Interest									3.1			16.0			(24.6)	24.2
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold															2,657.2	24.0
Net gain on sale of discontinued operations															414.5	18.2
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
Other Income															(21.0)
Assets									11.4						11.4			14.2
Liabilities									$ 7.2						7.2			30.3
Net Income (Loss) Attributable to Noncontrolling Interest															$ 20.3	(0.9)
WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of Business in the Insurance Market													80.00%
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
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%	54.00%		96.70%						96.70%
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									$ 0.4			(0.8)
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 | ₪								₪ 11.5						₪ 27.9
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									3.1						$ 7.5
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
Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets					5.5												$ 11.7
Goodwill					0.3
Intangible Assets, Net (Excluding Goodwill)					2.8												9.0
Liabilities					$ 1.2												$ 0.1
Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Other Investments	$ 161.8									161.8
Investments																		686.2
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 2,600.0									2,600.0
Net gain on sale of discontinued operations									(4.0)	366.6		0.0			362.6	0.0
Accumulated Other Comprehensive Income Loss Net Of Tax Other Period Increase Decrease									0.0	$ 113.3		(18.5)			32.0	(62.1)
Sale of Sirius Group [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accumulated Other Comprehensive Income Loss Net Of Tax Other Period Increase Decrease									0.0			$ 0.0	$ 113.3		$ 113.3	$ 0.0
Tranzact [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Repayments of Debt									56.3
Increase of Book Value									82.1
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold									221.3
Net gain on sale of discontinued operations									51.9
Discontinued Operation, Tax Effect of Discontinued Operation									$ 30.2
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%	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
Dividends Payable, Amount Per Share | $ / shares		$ 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