Summary of Significant Accounting Policies - Additional Information (Details)		12 Months Ended
	Jan. 18, 2017	Dec. 31, 2017 USD ($) Employees Subsidiary Clients	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jan. 04, 2017 shares
Summary Of Significant Accounting Policies [Line Items]
Number of inactive wholly owned subsidiaries | Subsidiary		2
Number of full-time equivalent employees | Employees		394
Cash and cash equivalents held at the Federal Reserve Bank		$ 101,900,000
Impaired long-lived assets		0	$ 0
Investment in FHLB stock		19,060,000	33,750,000
Impairment charges		0	0
Core deposit intangible amortization expense		394,000	239,000	$ 180,000
Stock split, conversion ratio	2.00
Additional share of common stock following stock split | shares						1
Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to retained earnings		900,000
Core Deposit
Summary Of Significant Accounting Policies [Line Items]
Core deposit intangible assets			6,300,000	900,000
Core deposit intangible amortization expense			$ 400,000	$ 200,000	$ 200,000
Federal Home Loan Bank Stock
Summary Of Significant Accounting Policies [Line Items]
Adjustments in carrying value		$ 0
Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives		3 years
Estimated useful lives of other intangible assets		7 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives		10 years
Estimated useful lives of other intangible assets		10 years
FFA
Summary Of Significant Accounting Policies [Line Items]
Approximate number of clients | Clients		1,500
Aggregate assets under management		$ 4,300,000,000