Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

No definition available.

Name:	us-gaap_NumberOfOperatingSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Segment Information (Narrative) (Details) - segment	6 Months Ended
Segment Information (Narrative) (Details) - segment	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Number of operating segments	4
Workers' Compensation
Segment Reporting Information [Line Items]
Worker's compensation SPC percentage ceded	100.00%
Syndicate 1729
Segment Reporting Information [Line Items]
Proportion of Capital Provided to support Lloyd's Syndicate 1729		62.00%	58.00%
Syndicate 6131
Segment Reporting Information [Line Items]
Proportion of Capital Provided to support Lloyd's Syndicate 1729		100.00%