Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Cash Flows) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	$ 501.5	$ 425.2
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of trading securities	(3,257.8)	(2,854.8)
Purchases of other invested assets	(33.2)	(73.1)
Sales of trading securities	2,932.2	2,953.6
Sales of other invested assets	37.3	85.5
Net cash paid for acquisitions	(1.2)	(141.5)
Other	(19.5)	(44.1)
Net cash used in investing activities	(342.2)	(74.4)
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid	(46.2)	(43.2)
Intercompany dividend paid	0.0	0.0
Proceeds from the exercise of stock options	3.1	7.4
Share repurchases	(139.5)	(246.4)
Repayment of other long-term debt	(0.2)	0.0
Proceeds from other long-term debt	0.0	3.9
Net cash used in financing activities	(182.8)	(278.3)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(23.5)	72.5
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	608.0	589.3
CASH AND CASH EQUIVALENTS, END OF PERIOD	584.5	661.8
Consolidating Adjustments [Member]
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	(429.1)	(624.1)
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of trading securities	0.0	0.0
Purchases of other invested assets	0.0	0.0
Sales of trading securities	0.0	0.0
Sales of other invested assets	0.0	0.0
Net cash paid for acquisitions	0.0	0.0
Other	0.0	0.0
Net cash used in investing activities	0.0	0.0
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid	0.0	0.0
Intercompany dividend paid	429.1	624.1
Proceeds from the exercise of stock options	0.0	0.0
Share repurchases	0.0	0.0
Repayment of other long-term debt	0.0
Proceeds from other long-term debt		0.0
Net cash used in financing activities	429.1	624.1
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	0.0	0.0
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	0.0	0.0
CASH AND CASH EQUIVALENTS, END OF PERIOD	0.0	0.0
Allied World Switzerland (Parent Guarantor) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	183.4	278.8
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of trading securities	0.0	0.0
Purchases of other invested assets	0.0	0.0
Sales of trading securities	0.0	0.0
Sales of other invested assets	0.0	0.0
Net cash paid for acquisitions	0.0	0.0
Other	0.0	0.0
Net cash used in investing activities	0.0	0.0
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid	(46.2)	(43.2)
Intercompany dividend paid	0.0	0.0
Proceeds from the exercise of stock options	3.1	7.4
Share repurchases	(139.5)	(246.4)
Repayment of other long-term debt	0.0
Proceeds from other long-term debt		0.0
Net cash used in financing activities	(182.6)	(282.2)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	0.8	(3.4)
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	21.8	32.6
CASH AND CASH EQUIVALENTS, END OF PERIOD	22.6	29.2
Allied World Bermuda (Subsidiary Issuer) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	194.7	298.6
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of trading securities	0.0	0.0
Purchases of other invested assets	0.0	0.0
Sales of trading securities	0.0	0.0
Sales of other invested assets	0.0	0.0
Net cash paid for acquisitions	0.0	0.0
Other	0.0	0.0
Net cash used in investing activities	0.0	0.0
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid	0.0	0.0
Intercompany dividend paid	(194.6)	(298.4)
Proceeds from the exercise of stock options	0.0	0.0
Share repurchases	0.0	0.0
Repayment of other long-term debt	0.0
Proceeds from other long-term debt		0.0
Net cash used in financing activities	(194.6)	(298.4)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	0.1	0.2
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	1.0	1.7
CASH AND CASH EQUIVALENTS, END OF PERIOD	1.1	1.9
Other Allied World Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	552.5	471.9
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of trading securities	(3,257.8)	(2,854.8)
Purchases of other invested assets	(33.2)	(73.1)
Sales of trading securities	2,932.2	2,953.6
Sales of other invested assets	37.3	85.5
Net cash paid for acquisitions	(1.2)	(141.5)
Other	(19.5)	(44.1)
Net cash used in investing activities	(342.2)	(74.4)
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid	0.0	0.0
Intercompany dividend paid	(234.5)	(325.7)
Proceeds from the exercise of stock options	0.0	0.0
Share repurchases	0.0	0.0
Repayment of other long-term debt	(0.2)
Proceeds from other long-term debt		3.9
Net cash used in financing activities	(234.7)	(321.8)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(24.4)	75.7
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	585.2	555.0
CASH AND CASH EQUIVALENTS, END OF PERIOD	$ 560.8	$ 630.7