Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Cash Flows) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	$ 621.4	$ 583.4
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of trading securities	(4,322.6)	(3,962.3)
Purchases of other invested assets	(58.5)	(110.9)
Sales of trading securities	4,595.8	3,888.4
Sales of other invested assets	99.2	160.3
Net cash paid for acquisitions	(1.2)	(141.5)
Return of investment in subsidiary	0.0
Other	(41.3)	(102.7)
Net cash provided by (used in) investing activities	271.4	(268.7)
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Dividends paid	(68.8)	(66.8)
Intercompany dividends paid	0.0	0.0
Proceeds to parent	0.0
Proceeds from the exercise of stock options	7.7	9.2
Share repurchases	(165.7)	(246.4)
Repayment of senior notes	(500.0)	0.0
Proceeds from other long-term debt	0.0	4.0
Repayment of other long-term debt	(0.2)	(0.2)
Net cash used in financing activities	(727.0)	(300.2)
NET INCREASE IN CASH AND CASH EQUIVALENTS	165.8	14.5
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	608.0	589.3
CASH AND CASH EQUIVALENTS, END OF PERIOD	773.8	603.8
Consolidating Adjustments
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	(763.9)	(712.8)
CASH FLOWS PROVIDED BY (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
Return of investment in subsidiary	(250.0)
Other	0.0	0.0
Net cash provided by (used in) investing activities	(250.0)	0.0
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Dividends paid	0.0	0.0
Intercompany dividends paid	763.9	712.8
Proceeds to parent	250.0
Proceeds from the exercise of stock options	0.0	0.0
Share repurchases	0.0	0.0
Repayment of senior notes	0.0
Proceeds from other long-term debt	0.0	0.0
Repayment of other long-term debt	0.0	0.0
Net cash used in financing activities	1,013.9	712.8
NET 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] | Reportable Legal Entities
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	219.8	302.0
CASH FLOWS PROVIDED BY (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
Return of investment in subsidiary	0.0
Other	0.0	0.0
Net cash provided by (used in) investing activities	0.0	0.0
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Dividends paid	(68.8)	(66.8)
Intercompany dividends paid	0.0	0.0
Proceeds to parent	0.0
Proceeds from the exercise of stock options	7.7	9.2
Share repurchases	(165.7)	(246.4)
Repayment of senior notes	0.0
Proceeds from other long-term debt	0.0	0.0
Repayment of other long-term debt	0.0	0.0
Net cash used in financing activities	(226.8)	(304.0)
NET INCREASE IN CASH AND CASH EQUIVALENTS	(7.0)	(2.0)
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	21.8	32.6
CASH AND CASH EQUIVALENTS, END OF PERIOD	14.8	30.6
Allied World Bermuda (Subsidiary Issuer) [Member] | Reportable Legal Entities
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	477.5	333.1
CASH FLOWS PROVIDED BY (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
Return of investment in subsidiary	250.0
Other	0.0	0.0
Net cash provided by (used in) investing activities	250.0	0.0
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Dividends paid	0.0	0.0
Intercompany dividends paid	(227.6)	(333.4)
Proceeds to parent	0.0
Proceeds from the exercise of stock options	0.0	0.0
Share repurchases	0.0	0.0
Repayment of senior notes	(500.0)
Proceeds from other long-term debt	0.0	0.0
Repayment of other long-term debt	0.0	0.0
Net cash used in financing activities	(727.6)	(333.4)
NET INCREASE IN CASH AND CASH EQUIVALENTS	(0.1)	(0.3)
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	1.0	1.7
CASH AND CASH EQUIVALENTS, END OF PERIOD	0.9	1.4
Other Allied World Subsidiaries [Member] | Reportable Legal Entities
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	688.0	661.1
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of trading securities	(4,322.6)	(3,962.3)
Purchases of other invested assets	(58.5)	(110.9)
Sales of trading securities	4,595.8	3,888.4
Sales of other invested assets	99.2	160.3
Net cash paid for acquisitions	(1.2)	(141.5)
Return of investment in subsidiary	0.0
Other	(41.3)	(102.7)
Net cash provided by (used in) investing activities	271.4	(268.7)
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Dividends paid	0.0	0.0
Intercompany dividends paid	(536.3)	(379.4)
Proceeds to parent	(250.0)
Proceeds from the exercise of stock options	0.0	0.0
Share repurchases	0.0	0.0
Repayment of senior notes	0.0
Proceeds from other long-term debt	0.0	4.0
Repayment of other long-term debt	(0.2)	(0.2)
Net cash used in financing activities	(786.5)	(375.6)
NET INCREASE IN CASH AND CASH EQUIVALENTS	172.9	16.8
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	585.2	555.0
CASH AND CASH EQUIVALENTS, END OF PERIOD	$ 758.1	$ 571.8