Capital Stock (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended
	Feb. 01, 2022	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	May 25, 2023	Dec. 31, 2022
Class of Stock [Line Items]
Shares outstanding (in shares)		105,011,371		105,011,371			104,442,023
Common shares issued (in shares)					46,913,370
Stock repurchased and retired (in shares)		494	3,280,042	55,922	5,039,784
Stock Repurchased and Retired During Period, Value		$ 0.0	$ 133.0	$ 1.9	$ 198.9
Common Stock, Shares, Issued		105,011,371		105,011,371			104,442,023
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Authorized						2,000,000
Accelerated Share Repurchases, Adjustment to Recorded Amount					$ 133.1
Shares repurchased, average cost per share (in dollars per share)					$ 40.59
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)							104,442,023
Stock repurchased and retired (in shares)					3,729,300
Common Stock, Shares, Issued							104,442,023
Class A Common Stock | Great Western Bank
Class of Stock [Line Items]
Number of shares issued as consideration (in shares)	46,900,000
Director [Member]
Class of Stock [Line Items]
Common shares issued (in shares)				54,414
Director [Member] | Class A Common Stock
Class of Stock [Line Items]
Dividend, Share-based Payment Arrangement, Paid-in-Kind				$ 1.2