Acquisitions, Goodwill and Intangible Assets (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, Impairment Loss	$ 44,700		$ 0
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)	$ 1,300
Goodwill		$ 0	44,695
Indefinite-lived intangibles		$ 0
Weighted average period to amortize acquired intangible assets		13 years
Commercial Accounts business unit [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			2,100
Aerospace & Programs business unit [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			9,700
Commercial Auto business units [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			21,300
E&S Casualty business unit [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			6,300
Public Entity Component [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			2,600
E & S Package component [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			3,700
Personal Lines Business Unit [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			$ 5,300