Capital Stock (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended
	Feb. 01, 2022	Sep. 12, 2020	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021	Jun. 11, 2019
Class of Stock [Line Items]
Shares outstanding (in shares)			109,503,010		62,200,456
Common shares issued (in shares)			46,887,104	604
Stock Repurchase Program, Number of Shares Authorized to be Repurchased		5,500,000
Incremental Common Shares Attributable to Dilutive Effect of Accelerated Share Repurchase Agreements		3,000,000
Stock repurchased and retired (in shares)			37,698	127,180
Stock Repurchased and Retired During Period, Value			$ 1.4	$ 5.4
Shares repurchased during the period as percentage of shares authorized for repurchase (percent)				1.30%
Great Western Bank
Class of Stock [Line Items]
Total consideration exchanged	$ 1,723.3
Class A Common Stock
Class of Stock [Line Items]
Shares outstanding (in shares)					41,699,409
Stock Repurchase Program, Number of Shares Authorized to be Repurchased						2,500,000
Stock repurchased and retired (in shares)			0	72,700
Stock Repurchased and Retired During Period, Value				$ 2.9
Shares repurchased, average cost per share (in dollars per share)				$ 39.69
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased			1,900,000
Class A Common Stock | Great Western Bank
Class of Stock [Line Items]
Number of shares issued as consideration (in shares)	46,879,601
Class B Common Stock
Class of Stock [Line Items]
Shares outstanding (in shares)					20,501,047
Director [Member]
Class of Stock [Line Items]
Common shares issued (in shares)			7,503	604