Acquisitions (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Apr. 01, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Goodwill	$ 472.9	$ 1.6	$ 472.9	$ 1.6		$ 1.6
Revenues	5,283.3	4,741.5	10,178.6	9,449.1
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 368.5	$ 293.4	$ 664.1	$ 614.7
ARX Holding Corp. [Member]
Business Acquisition [Line Items]
Interest acquired (percent)					63.20%
Additional interest acquired (percent)	0.90%
Cost of interest acquired	$ 890.0
Ownership interest held prior to acquisition (percent)					5.00%
Loss recognized to reflect the net acquisition cost attributable to interest held prior	$ 2.0
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage	69.10%		69.10%
Goodwill					$ 470.0
Value at acquisition	$ 520.4		$ 520.4
Total assets acquired					1,800.0
Investment securities acquired					1,200.0
Cash and cash equivalents acquired					183.0
Prepaid reinsurance premiums acquired					146.0
Libilities assumed in acquisition					1,200.0
Unearned premiums acquired					550.0
Loss and loss adjustment expense reserves acquired					306.0
Debt acquired					$ 185.0
Revenues	208.2
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 16.8
Sales Revenue, Segment [Member] | ARX Holding Corp. [Member]
Business Acquisition [Line Items]
Percentage of total net premiums written (percent)	5.00%