Segment Information (Schedule of Additional Business Segment Information) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Assets	$ 1,244,245	$ 1,231,126	$ 1,244,245	$ 1,231,126
Specialty Commercial Segment [Member]
Segment Reporting Information [Line Items]
Assets			822,082	810,133
Standard Commercial Segment [Member]
Segment Reporting Information [Line Items]
Assets			163,157	162,152
Personal Segment [Member]
Segment Reporting Information [Line Items]
Assets			231,242	232,441
Corporate [Member]
Segment Reporting Information [Line Items]
Assets			$ 27,764	$ 26,400