Basis of Presentation and Significant Accounting Policies - Summary of Common Shares Issued and Resultant Value of Total Shareholders' Equity (Detail) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Total Shareholders' Equity		$ 300,828
Common Stock [Member]
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Total Shareholders' Equity		16,527
Minnesota-based Klein [Member]
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Total Shareholders' Equity	$ 406,474
Minnesota-based Klein [Member] | Common Stock [Member]
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Shares of Common Stock	22,772
Acquisition, Total Shareholders' Equity	$ 22,772
Anchor Bank (MN) [Member]
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Total Shareholders' Equity		$ 300,828
Anchor Bank (MN) [Member] | Common Stock [Member]
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Shares of Common Stock		16,527