Significant Transactions (Details) $ / shares in Units, ₪ in Millions, $ in Millions				1 Months Ended	2 Months Ended				3 Months Ended							12 Months Ended
	Aug. 04, 2016 USD ($)	Apr. 18, 2016 USD ($)	Feb. 01, 2016 USD ($) $ / shares	Feb. 01, 2016 $ / shares	Feb. 23, 2015 ILS (₪)	Feb. 23, 2015 USD ($)	Feb. 19, 2014 ILS (₪)	Feb. 19, 2014 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2016 ILS (₪)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 ILS (₪)	Dec. 31, 2015 USD ($)	Mar. 14, 2014 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Repayments of Debt									$ 1.2					$ 0.0
Payments to Acquire Other Investments									22.4					10.9
Investments									5,339.0	$ 5,334.8							$ 5,334.8
Sales of common equity securities									17.7					767.5
Payments to Acquire Businesses and Interest in Affiliates									0.0					8.1
Other Income									43.7					39.8
Assets									6,516.9	6,544.7							6,544.7
Goodwill									31.7	31.7			$ 24.1	24.1			31.7		$ 24.1
Intangible Assets, Net (Excluding Goodwill)									21.3	24.2			32.2	32.2			24.2		$ 31.3
Liabilities									2,759.3	2,807.5							2,807.5
Net Income (Loss) Attributable to Noncontrolling Interest									1.3					(6.3)
Net (loss) gain on sale of discontinued operations, net of tax									$ (1.0)					0.0
Buzzmove [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)	70.90%
Assets	$ 11.5
Goodwill	7.6
Intangible Assets, Net (Excluding Goodwill)	1.1
Liabilities	0.1
Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)						56.20%
Goodwill						$ 0.3
Liabilities						1.2
WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)									95.00%
Goodwill								$ 5.5
Liabilities								$ 0.7
QuoteLab [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)																				60.00%
Payments to Acquire Interest in Subsidiaries and Affiliates															$ 28.1
Assets																				$ 70.1
Goodwill																				18.3
Intangible Assets, Net (Excluding Goodwill)																				38.5
Liabilities																				$ 10.0
Percent of gross profit																		62.50%	62.50%
Additional cash that will be paid as part of the purchase price													7.8	7.8
LowFares.com
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	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%
United Kingdom, Pounds | Buzzmove [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates	6.1
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 | ₪																₪ 35.9
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											₪ 79.6
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 | ₪					₪ 9.5													26.4
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											₪ 11.8
United States of America, Dollars | Buzzmove [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates	$ 8.1
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																	9.6
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											$ 20.7
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						2.4													6.5
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											3.1
WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets						5.5		13.4
Intangible Assets, Net (Excluding Goodwill)						$ 2.8		$ 2.9
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 (loss) gain on sale of discontinued operations, net of tax											$ 4.0						363.2
Sale of Sirius Group [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Change in foreign currency translation and other, net of tax																	113.3
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										$ 1.2	221.3
Net (loss) gain on sale of discontinued operations, net of tax									$ (1.0)		51.9						51.9
Discontinued Operation, Tax Effect of Discontinued Operation											$ 30.2						$ 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	$ 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
Common Stock, Dividends, Per Share, Cash Paid | $ / shares				$ 0.50
Symetra | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investments																			$ 528.6