Material Transactions Affecting Stockholders' Equity (Details) - $ / shares				3 Months Ended
	May 17, 2017	Jan. 06, 2017	Mar. 08, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015
Class of Stock [Line Items]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01	$ 0.01				$ 0.01
Common stock, increase shares authorized (in shares)	100,000,000
Common stock and preferred stock, shares authorized (in shares)	251,000,000
Preferred stock, shares authorized (in shares)	1,000,000			1,000,000				1,000,000
Common stock, shares authorized (in shares)	250,000,000			250,000,000				150,000,000
Common dividends declared per common share (in dollars per share)				$ 0.10	$ 0.10	$ 0.10	$ 0.09	$ 0.09	$ 0.09	$ 0.09	$ 0.09	$ 0.09	$ 0.09	$ 0.09	$ 0.09
Standard Bancshares, Inc.
Class of Stock [Line Items]
Common stock, shares issued (in shares)		21,057,085
Shares issued (in dollars per share)		$ 25.34
NI Bancshares Corporation
Class of Stock [Line Items]
Common stock, shares issued (in shares)			3,042,494
Shares issued (in dollars per share)			$ 18.059
Standard Bancshares, Inc.
Class of Stock [Line Items]
Shares issued (in dollars per share)		$ 25.34