Capital Stock (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended
	Feb. 01, 2022	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	May 25, 2022	Dec. 31, 2021
Class of Stock [Line Items]
Shares outstanding (in shares)		104,450,804		104,450,804			62,200,456
Common shares issued (in shares)			1,782	46,913,370	19,081
Stock repurchased and retired (in shares)		3,280,042	574	5,039,784	128,038
Stock Repurchased and Retired During Period, Value		$ 133.0	$ 0.0	$ 198.9	$ 5.4
Shares repurchased, average cost per share (in dollars per share)		$ 40.59		$ 39.48
Accelerated Share Repurchases, Adjustment to Recorded Amount		$ 133.1		$ 197.4
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						5,000,000
Stock repurchased and retired (in shares)		3,279,300
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased						1,889,158
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)				33,769	19,081