Capital Stock and Dividend Restrictions - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Feb. 01, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	May 25, 2022	Dec. 14, 2021	Jun. 11, 2019
Class of Stock
Common stock, shares authorized		250,000,000	100,000,000
Common stock, shares, outstanding		104,442,023	62,200,456
Stock compensation expense		$ 9.6	$ 8.9	$ 7.5
Aggregate purchase price of shares repurchased and retired		$ 199.0	$ 5.4	$ 116.8
Stock Repurchased and Retired During Period, Shares		5,040,896	128,171	3,578,743
Sale of Stock Shares Authorized For Future Sale						47,158,390
Accelerated Share Repurchases, Adjustment to Recorded Amount		$ 197.4
Treasury Stock Acquired, Average Cost Per Share		$ 39.48
Great Western Bank
Class of Stock
Total consideration paid	$ 1,723.3
Class A Common Stock
Class of Stock
Stock Repurchased and Retired During Period, Shares		5,000,000
Class A Common Stock
Class of Stock
Common stock, shares authorized		150,000,000
Common Stock, Voting Rights, Votes Per Share		$ 1
Common stock, shares, outstanding			41,699,409
Stock Repurchase Program, Number of Shares Authorized to be Repurchased							5,000,000
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased					1,889,158
Class A Common Stock | Great Western Bank
Class of Stock
Common stock issued (in shares)	46,879,601
Class B Common Stock
Class of Stock
Common stock, shares authorized		100,000,000
Common stock, shares, outstanding		0	20,501,047
Director [Member] | Class A Common Stock
Class of Stock
Shares issued to directors during period		33,769
Stock compensation expense		$ 1.3
Board of Directors
Class of Stock
Aggregate value of shares issued to directors			$ 0.9
Board of Directors | Class A Common Stock
Class of Stock
Shares issued to directors during period			19,081