Significant Transactions (Details) $ / shares in Units, ₪ in Millions, $ in Millions	1 Months Ended			2 Months Ended				3 Months Ended						9 Months Ended
	Sep. 30, 2015 USD ($) $ / shares	Jan. 31, 2014 USD ($)	Oct. 31, 2011 USD ($)	Feb. 23, 2015 ILS (₪)	Feb. 23, 2015 USD ($)	Feb. 19, 2014 ILS (₪) business	Feb. 19, 2014 USD ($) business	Sep. 30, 2015 USD ($) $ / shares	Jun. 30, 2015 ILS (₪)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Sep. 30, 2015 USD ($) $ / shares	Sep. 30, 2014 USD ($)	Sep. 01, 2015 USD ($)	Jul. 27, 2015 USD ($)	Oct. 10, 2014 USD ($)	Jul. 01, 2014	Jun. 30, 2014 USD ($)	Mar. 14, 2014 USD ($)	Dec. 31, 2013 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investments	$ 3,709.5							$ 3,709.5			$ 3,742.4			$ 3,709.5
Loss and loss adjustment expenses								170.0				$ 185.5		534.1	$ 527.3
Payments to Acquire Businesses and Interest in Affiliates														33.3	32.2
Assets	10,346.0							10,346.0			10,455.7			10,346.0
Goodwill	189.3							189.3		$ 169.2	168.9	23.8		189.3	23.8					$ 23.8		$ 0.0
Intangible Assets, Net (Excluding Goodwill)	192.4							192.4		$ 171.2	182.3	42.0		192.4	42.0					$ 44.6		$ 5.1
Goodwill and intangible assets	381.7							381.7			351.2			381.7
Liabilities	6,139.7							6,139.7			5,917.3			6,139.7
Payments to Acquire Additional Interest in Subsidiaries														9.1	0.0
Net Income (Loss) Attributable to Noncontrolling Interest								16.0				11.2		24.2	7.2
QuoteLab
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business Combination, Contingent Consideration, Liability											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
Enterprise Value of Subsidiary																		$ 281.2
Proceeds from Contributions from Affiliates											44.2
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]
Enterprise Value of Subsidiary																$ 50.2
PPCI Global Ltd. [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share										50
Payments to Acquire Interest in Subsidiaries and Affiliates										$ 21.0
OneTitle [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Agreed Upon Commitment of Contributed Capital										13.0
Star & Shield Insurance Exchange
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase of Surplus Notes	20.0							20.0						20.0
Assets	16.3							16.3			13.5			16.3
Liabilities	$ 31.5							$ 31.5			$ 25.9			$ 31.5
MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business Combination, Contingent Consideration, Liability										$ 7.9
Payments to Acquire Businesses and Interest in Affiliates													$ 28.1
Percentage of gross profit																					62.50%
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]
Premiums Written, Percentage of Total						85.00%	85.00%
Number of lines of business | business						4	4
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
Common Class A | durchblicker.at [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)																			45.00%
Common Class A | MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)																					60.00%
Common Stock | TruBridge [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)																100.00%
Common Stock | Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)					56.20%					68.30%
Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)	95.30%						54.00%	95.30%						95.30%
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								$ (0.9)				1.4		$ (2.8)	$ 11.1
Common Stock | TruBridge [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates	$ 31.0
Euro Member Countries, Euro | Common Stock | durchblicker.at [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates												8.5
Israel, New Shekels
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates | ₪									₪ 10.0
Israel, New Shekels | Convertible Preferred Stock [Member] | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Additional Interest in Subsidiaries | ₪									25.0
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 | ₪									₪ 35.0
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 | ₪				₪ 9.5
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
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates										$ 2.6
United States of America, Dollars | Convertible Preferred Stock [Member] | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Additional Interest in Subsidiaries										6.6
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										$ 9.1
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							$ 8.5
United States of America, Dollars | Common Stock | durchblicker.at [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates												$ 11.7
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
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
TruBridge [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets																$ 53.8
Goodwill																20.1
Intangible Assets, Net (Excluding Goodwill)																28.1
Liabilities																$ 3.6
Goodwill, Purchase Accounting Adjustments														9.7
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]
Percentage of common shareholder's equity																	127.30%
Investments	510.8							510.8						510.8
Sale of Sirius [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Additional cash that will be paid as part of the purchase price																	$ 10.0
Crop insurance [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ceded Premiums Written								35.9
Ceded Premiums Earned								16.8
Loss and loss adjustment expenses								15.1
Ceded insurance Acquisition Expenses								2.8
Proceeds related from termination of agreement	$ 3.0							$ 3.0						$ 3.0
Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)	17.70%							17.70%			17.31%			17.70%
Dividends | $ / shares								$ 0.50
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						$ 32.00