Significant Transactions (Details) $ / shares in Units, ₪ in Millions, $ in Millions			1 Months Ended		2 Months Ended				3 Months Ended						6 Months Ended
	Apr. 18, 2016 USD ($)	Feb. 01, 2016 USD ($)	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 ILS (₪)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($) $ / shares	Mar. 31, 2016 USD ($) $ / shares	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jun. 30, 2016 ILS (₪)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Oct. 10, 2014	Mar. 31, 2014 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Other Investments																$ 13.3	$ 28.6
Investments										$ 5,521.3						5,521.3		$ 4,271.1
Sales of common equity securities																851.3	306.8
Loss and loss adjustment expenses										182.0			$ 196.2			343.1	364.1
Payments to Acquire Businesses and Interest in Affiliates																8.1	2.4
Other Income										36.2			48.7			76.0	76.1
Assets										7,065.6						7,065.6		10,282.6
Goodwill										24.1	$ 24.1	$ 24.1	24.1	$ 23.8		24.1	24.1	24.1	$ 24.1
Intangible Assets, Net (Excluding Goodwill)										29.2	32.2	$ 32.2	36.4	38.7		29.2	36.4	31.3	$ 39.0
Goodwill and intangible assets										53.3						53.3		55.4
Liabilities										2,966.2						2,966.2		5,914.6
Net Income (Loss) Attributable to Noncontrolling Interest										(21.4)			0.9			(27.7)	8.2
Net gain on sale of discontinued operations										366.6			(0.1)			366.6	7.9
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
Proceeds from Contributions from Affiliates														$ 44.2
Ownership interest (as a percent)																				63.20%
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
Surplus Notes										1.0						1.0
Other Income																(20.0)
Assets										12.3						12.3		14.2
Liabilities										$ 8.5						8.5		30.3
Net Income (Loss) Attributable to Noncontrolling Interest																$ 19.9	(1.9)
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%	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
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
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%		96.50%						96.50%
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										$ 20.0			(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 | ₪									₪ 16.4						₪ 16.4
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										4.4						$ 4.4
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
Goodwill						0.3
Intangible Assets, Net (Excluding Goodwill)						2.8
Liabilities						$ 1.2
Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Increase of Book Value																479.9
Payments to Acquire Other Investments	$ 161.8									161.8
Investments																		686.2
Net gain on sale of discontinued operations																366.6
Accumulated Other Comprehensive Income Loss Net Of Tax Other Period Increase Decrease										(5.2)			44.0			32.0	(43.6)
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										$ 113.3			$ 0.0			$ 113.3	$ 0.0
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
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