Reinsurance - Additional Information (Detail) (GBP £) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Number of companies that make up reinsurance recoverable	85
Number of companies that make up majority of reinsurance recoverable	13
Percent of reinsurance recoverable made up by majority of companies	92.00%
Remaining companies rated A- or better by AM best	7.00%
Percent of recoverable held with companies less than A-	1.00%
Reinsurance Agreement [Member] | Minimum [Member]
Quota Share Reinsurance Percentage	50.00%
Quota Share Coverage Amount	£ 0.5
Reinsurance Agreement [Member] | Maximum [Member]
Quota Share Reinsurance Percentage	100.00%