Amount before tax of foreign currency transaction realized and unrealized gain recognized in the income statement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 830

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6450189&loc=d3e30690-110894

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 830

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6450222&loc=d3e30840-110895

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 830

-SubTopic 20

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6450189&loc=d3e30700-110894

Name:	us-gaap_ForeignCurrencyTransactionGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Lloyd's Syndicate 1221 - Additional Information (Detail) (Lloyd's, Syndicate 1221) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Mar. 31, 2014 USD ($)	Sep. 30, 2014 GBP (£)	Sep. 30, 2013 GBP (£)	Dec. 31, 2013	Dec. 31, 2012
Supplementary Insurance Information, by Segment [Line Items]
Syndicate 1221's stamp capacity		£ 215	£ 195
Percentage control of stamp capacity		100.00%		100.00%	100.00%
Pre-tax gain in connection with a change in functional currency	10.0
After-tax gain in connection with a change in functional currency	$ 6.6