Gross Written Premium by Geographical Location (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Gross written premium, percentage	100.00%	100.00%
UNITED STATES
Segment Reporting Information [Line Items]
Gross written premium, percentage	76.00%	87.00%
U.K.
Segment Reporting Information [Line Items]
Gross written premium, percentage	6.00%	4.00%
Worldwide And Other
Segment Reporting Information [Line Items]
Gross written premium, percentage	18.00%	9.00%