Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Cash Flows) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	$ 196.6	$ 349.3
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of trading securities	(779.8)	(1,530.0)
Purchases of other invested assets	(11.4)	(8.1)
Sales of trading securities	1,141.9	1,414.6
Sales of other invested assets	0.5	38.7
Other	14.9	(27.4)
Net cash provided by (used in) investing activities	366.1	(112.2)
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid	0.0	(23.4)
Intercompany dividends paid	0.0	0.0
Proceeds from the exercise of stock options	1.4	0.4
Repayment of other long-term debt	(0.1)	(0.1)
Share repurchases	0.0	(50.0)
Net cash used in financing activities	1.3	(73.1)
NET INCREASE IN CASH AND CASH EQUIVALENTS	564.0	164.0
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	720.9	608.0
CASH AND CASH EQUIVALENTS, END OF PERIOD	1,284.9	772.0
Consolidation, Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	(20.0)	(176.7)
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
Other	0.0	0.0
Net cash provided by (used in) investing activities	0.0	0.0
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid	0.0	0.0
Intercompany dividends paid	20.0	176.7
Proceeds from the exercise of stock options	0.0	0.0
Repayment of other long-term debt	0.0	0.0
Share repurchases		0.0
Net cash used in financing activities	20.0	176.7
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]
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	0.3	69.7
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
Other	0.0	0.0
Net cash provided by (used in) investing activities	0.0	0.0
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid	0.0	(23.4)
Intercompany dividends paid	0.0	0.0
Proceeds from the exercise of stock options	1.4	0.4
Repayment of other long-term debt	0.0	0.0
Share repurchases		(50.0)
Net cash used in financing activities	1.4	(73.0)
NET INCREASE IN CASH AND CASH EQUIVALENTS	1.7	(3.3)
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	13.1	21.8
CASH AND CASH EQUIVALENTS, END OF PERIOD	14.8	18.5
Allied World Bermuda (Subsidiary Issuer) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	35.0	78.9
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
Other	0.0	0.0
Net cash provided by (used in) investing activities	0.0	0.0
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid	0.0	0.0
Intercompany dividends paid	(10.0)	(78.5)
Proceeds from the exercise of stock options	0.0	0.0
Repayment of other long-term debt	0.0	0.0
Share repurchases		0.0
Net cash used in financing activities	(10.0)	(78.5)
NET INCREASE IN CASH AND CASH EQUIVALENTS	25.0	0.4
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	0.8	1.0
CASH AND CASH EQUIVALENTS, END OF PERIOD	25.8	1.4
Other Allied World Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	181.3	377.4
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of trading securities	(779.8)	(1,530.0)
Purchases of other invested assets	(11.4)	(8.1)
Sales of trading securities	1,141.9	1,414.6
Sales of other invested assets	0.5	38.7
Other	14.9	(27.4)
Net cash provided by (used in) investing activities	366.1	(112.2)
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid	0.0	0.0
Intercompany dividends paid	(10.0)	(98.2)
Proceeds from the exercise of stock options	0.0	0.0
Repayment of other long-term debt	(0.1)	(0.1)
Share repurchases		0.0
Net cash used in financing activities	(10.1)	(98.3)
NET INCREASE IN CASH AND CASH EQUIVALENTS	537.3	166.9
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	707.0	585.2
CASH AND CASH EQUIVALENTS, END OF PERIOD	$ 1,244.3	$ 752.1