Acquisition of Business	6 Months Ended
Jun. 30, 2020
Business Combinations [Abstract]
Acquisition of Business	Acquisition of BusinessesEffective in June 2019, National Interstate, a property and casualty insurance subsidiary of AFG, entered into an agreement with Atlas Financial Holdings, Inc. (“AFH”) to become the exclusive underwriter of AFH’s paratransit book of business. National Interstate estimates that the majority of AFH’s $110 million paratransit business will be eligible for quotation under this arrangement following inception of the agreement. Under the terms of the agreement (as extended in 2020), AFH will act as an underwriting manager for National Interstate until at least August 2021 for fleets with seven or fewer vehicles and until November 2020 for accounts with eight or more vehicles, after which time National Interstate is entitled to acquire the renewal rights for the business from AFH for a purchase price equal to 15% of the in force gross written premiums at that date. The majority of the purchase price ultimately paid for the renewal rights will be recorded as an intangible renewal rights asset and will be amortized over the estimated life of the business acquired. In connection with the transaction, AFG was granted a five-year warrant to acquire approximately 2.4 million shares of AFH (19.9% at the acquisition date). The estimated fair value of the warrant was approximately $1 million at the date it was received.On June 30, 2020, AFG acquired 100% of the indirect noncontrolling interest in Neon, its United Kingdom-based Lloyd’s insurer from certain former and current Neon executives for cash based on the nominal fair value of the interest acquired as determined by a third-party valuation firm.

Acquisition of Business

6 Months Ended

Jun. 30, 2020

Acquisition of BusinessesEffective in June 2019, National Interstate, a property and casualty insurance subsidiary of AFG, entered into an agreement with Atlas Financial Holdings, Inc. (“AFH”) to become the exclusive underwriter of AFH’s paratransit book of business. National Interstate estimates that the majority of AFH’s $110 million paratransit business will be eligible for quotation under this arrangement following inception of the agreement. Under the terms of the agreement (as extended in 2020), AFH will act as an underwriting manager for National Interstate until at least August 2021 for fleets with seven or fewer vehicles and until November 2020 for accounts with eight or more vehicles, after which time National Interstate is entitled to acquire the renewal rights for the business from AFH for a purchase price equal to 15% of the in force gross written premiums at that date. The majority of the purchase price ultimately paid for the renewal rights will be recorded as an intangible renewal rights asset and will be amortized over the estimated life of the business acquired. In connection with the transaction, AFG was granted a five-year warrant to acquire approximately 2.4 million shares of AFH (19.9% at the acquisition date). The estimated fair value of the warrant was approximately $1 million at the date it was received.On June 30, 2020, AFG acquired 100% of the indirect noncontrolling interest in Neon, its United Kingdom-based Lloyd’s insurer from certain former and current Neon executives for cash based on the nominal fair value of the interest acquired as determined by a third-party valuation firm.