Underwriting Information - Reportable Segments (Details) - segment	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Reportable segment
Number of reportable segments	1	1

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

No definition available.

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