Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Cash Flows) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	$ 349.3	$ 312.7
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of trading securities	(1,530.0)	(1,723.4)
Purchases of other invested assets	(8.1)	(233.3)
Sales of trading securities	1,414.6	1,561.9
Sales of other invested assets	38.7	56.5
Other	(27.4)	19.1
Net cash used in investing activities	(112.2)	(319.2)
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid	(23.4)	(21.7)
Intercompany dividends paid	0.0	0.0
Proceeds from the exercise of stock options	0.4	4.2
Repayment of other long-term debt	(0.1)	0.0
Share repurchases	(50.0)	(50.3)
Net cash used in financing activities	(73.1)	(67.8)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	164.0	(74.3)
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	608.0	589.3
CASH AND CASH EQUIVALENTS, END OF PERIOD	772.0	515.0
Consolidation, Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	(176.7)	(179.0)
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
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 dividends paid	176.7	179.0
Proceeds from the exercise of stock options	0.0	0.0
Repayment of other long-term debt	0.0
Share repurchases	0.0	0.0
Net cash used in financing activities	176.7	179.0
NET INCREASE (DECREASE) 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:	69.7	71.6
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
Other	0.0	0.0
Net cash used in investing activities	0.0	0.0
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid	(23.4)	(21.7)
Intercompany dividends paid	0.0	0.0
Proceeds from the exercise of stock options	0.4	4.2
Repayment of other long-term debt	0.0
Share repurchases	(50.0)	(50.3)
Net cash used in financing activities	(73.0)	(67.8)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(3.3)	3.8
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	21.8	32.6
CASH AND CASH EQUIVALENTS, END OF PERIOD	18.5	36.4
Allied World Bermuda (Subsidiary Issuer) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	78.9	80.4
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
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 dividends paid	(78.5)	(80.0)
Proceeds from the exercise of stock options	0.0	0.0
Repayment of other long-term debt	0.0
Share repurchases	0.0	0.0
Net cash used in financing activities	(78.5)	(80.0)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	0.4	0.4
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	1.0	1.7
CASH AND CASH EQUIVALENTS, END OF PERIOD	1.4	2.1
Other Allied World Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	377.4	339.7
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of trading securities	(1,530.0)	(1,723.4)
Purchases of other invested assets	(8.1)	(233.3)
Sales of trading securities	1,414.6	1,561.9
Sales of other invested assets	38.7	56.5
Other	(27.4)	19.1
Net cash used in investing activities	(112.2)	(319.2)
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid	0.0	0.0
Intercompany dividends paid	(98.2)	(99.0)
Proceeds from the exercise of stock options	0.0	0.0
Repayment of other long-term debt	(0.1)
Share repurchases	0.0	0.0
Net cash used in financing activities	(98.3)	(99.0)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	166.9	(78.5)
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	585.2	555.0
CASH AND CASH EQUIVALENTS, END OF PERIOD	$ 752.1	$ 476.5