Accounting for Certain Loans Acquired in a Purchase - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance amount of loans acquired	$ 1,302,134,000	$ 698,097,000
Carrying amount of loans acquired	1,257,617,000	665,037,000
Loans acquired with deteriorated credit quality
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance amount of loans acquired	38,900,000	60,500,000
Carrying amount of loans acquired	25,300,000	43,200,000
Allowance for loan losses and provision expense	$ 0	0
Loans acquired with deteriorated credit quality | Transferred to Other Real Estate Owned
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance amount of loans acquired		12,100,000
Carrying amount of loans acquired		$ 7,600,000
Loans transferred to Other Real Estate Owned, number | loan		1