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, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Total Shareholders' Equity	$ 300,828	$ 273,565
Anchor Bank (MN) [Member]
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Total Shareholders' Equity	300,828
AnchorBank WI [Member]
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Total Shareholders' Equity		273,565
Founders Financial Corporation [Member]
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Total Shareholders' Equity			$ 50,626
Common Stock [Member]
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Total Shareholders' Equity	$ 16,527	$ 20,415
Common Stock [Member] | Anchor Bank (MN) [Member]
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Shares of Common Stock	16,527
Common Stock [Member] | AnchorBank WI [Member]
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Shares of Common Stock		20,415
Common Stock [Member] | Founders Financial Corporation [Member]
Common Shares Issued and Value of Shares Holders' Equity [Line Items]
Acquisition, Shares of Common Stock			3,402
Acquisition, Total Shareholders' Equity			$ 3,402