Capital Stock and Dividend Restrictions - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions			12 Months Ended
	Dec. 14, 2023	Feb. 01, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	May 25, 2023	Mar. 24, 2022	Dec. 14, 2021
Class of Stock
Common stock, shares authorized			150,000,000				250,000,000
Common stock, shares, outstanding			103,941,626	104,442,023
Stock compensation expense			$ 4.7	$ 9.6	$ 8.9
Payments for Repurchase of Common Stock	$ 32.1		$ 34.0	$ 199.0	$ 5.4
Stock Repurchased and Retired During Period, Shares			1,056,191	5,040,896	128,171
Sale of Stock Shares Authorized For Future Sale								47,158,390
Preferred Stock, Shares Authorized			100,000	100,000
Capital Stock, Authorized			150,100,000
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Authorized						2,000,000,000,000
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				5,000,000,000,000
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				104,442,023
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
Director [Member] | Class A Common Stock
Class of Stock
Shares issued to directors during period			54,414
Stock compensation expense			$ 1.2
Board of Directors
Class of Stock
Aggregate value of shares issued to directors				$ 1.3
Board of Directors | Class A Common Stock
Class of Stock
Shares issued to directors during period				33,769