Capital Stock (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended
	Feb. 01, 2022	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Class of Stock [Line Items]
Shares outstanding (in shares)		105,020,683		105,020,683		104,442,023
Common shares issued (in shares)		54,414	26,266	54,414	46,913,370
Stock repurchased and retired (in shares)		348	1,722,044	55,428	1,759,742
Stock Repurchased and Retired During Period, Value		$ 0.0	$ 64.5	$ 1.8	$ 65.9
Common Stock, Shares, Issued		105,020,683		105,020,683		104,442,023
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Authorized		2,000,000		2,000,000
Accelerated Share Repurchases, Adjustment to Recorded Amount					$ 64.3
Shares repurchased, average cost per share (in dollars per share)					$ 37.38
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)					1,720,700
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