Summary of Significant Accounting Policies - Intangible assets, Transfer of financial assets (Details)		12 Months Ended
	Apr. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) item
Intangible Assets
Number of reportable operating segments | item		2
Impairment Charges	$ 0	$ 0
Excess tax benefits recorded against income tax expense		$ 1,547,000
Minimum
Intangible Assets
Estimated useful lives of intangibles		2 years
Maximum
Intangible Assets
Estimated useful lives of intangibles		15 years
Core deposit intangibles | Minimum
Intangible Assets
Estimated useful lives of intangibles		10 years
Core deposit intangibles | Maximum
Intangible Assets
Estimated useful lives of intangibles		13 years
Noncompete intangibles | Minimum
Intangible Assets
Estimated useful lives of intangibles		2 years
Noncompete intangibles | Maximum
Intangible Assets
Estimated useful lives of intangibles		3 years
Client list intangibles
Intangible Assets
Estimated useful lives of intangibles		15 years