Capital Stock (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended				6 Months Ended			9 Months Ended	12 Months Ended
	Apr. 08, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2019	Mar. 31, 2018	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018	Dec. 31, 2019	Jun. 11, 2019
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value		$ 0.0	$ 0.1			$ 32.6	$ 2.5
Stock repurchased and retired (in shares)				607	39,360		896	22,596
Common Stock, Par or Stated Value Per Share		$ 29.56				$ 29.56
Class A Common Stock
Class of Stock [Line Items]
Shares outstanding (in shares)		42,579,590				42,579,590				43,129,085
Stock Repurchased and Retired During Period, Value						$ 31.2
Stock repurchased and retired (in shares)		0				1,054,700			0	0
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased		1,445,300				1,445,300
Stock Repurchase Program, Number of Shares Authorized to be Repurchased											2,500,000
Class B Common Stock
Class of Stock [Line Items]
Shares outstanding (in shares)		21,981,114				21,981,114				22,117,254
Idaho Independent Bank
Class of Stock [Line Items]
Value of common stock consideration issued	$ 157.3
Idaho Independent Bank | Class A Common Stock
Class of Stock [Line Items]
Number of shares issued as consideration (in shares)	3,871,422
Community 1st Bank | Class A Common Stock
Class of Stock [Line Items]
Number of shares issued as consideration (in shares)	463,134
Director [Member] | Class A Common Stock
Class of Stock [Line Items]
Common Stock, Shares, Issued		19,200				19,200
Dividend, Share-based Payment Arrangement, Paid-in-Kind						$ 0.6