Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions			12 Months Ended
	Dec. 14, 2023	Feb. 01, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Cash flows from operating activities:
Net income			$ 226.0	$ 257.5	$ 202.2
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation expense			11.8	4.7	9.6
Net cash provided by operating activities			355.0	428.0	534.4
Cash flows from investing activities:
Net cash used in investing activities			1,758.8	1,253.4	(949.0)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of stock issuance costs			0.0	0.0	0.1
Purchase and retirement of common stock	$ (32.1)		(1.2)	(34.0)	(199.0)
Dividends paid to common stockholders			(195.9)	(195.1)	(182.1)
Net cash used in financing activities			(1,795.2)	(1,973.9)	(1,059.7)
Net increase (decrease) in cash and cash equivalents			318.6	(292.5)	(1,474.3)
Cash and cash equivalents at beginning of period			578.0	870.5	2,344.8
Cash and cash equivalents at end of period			896.6	578.0	870.5
Parent Company
Cash flows from operating activities:
Net income			226.0	257.5	202.2
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries			(49.1)	83.4	88.3
Stock-based compensation expense			11.9	4.6	9.6
Other, net			1.4	2.7	(151.2)
Net cash provided by operating activities			190.2	348.2	148.9
Cash flows from investing activities:
Acquisition of bank and bank holding company, net of cash and cash equivalents received			0.0	0.0	(0.8)
Net cash used in investing activities			0.0	0.0	(0.8)
Cash flows from financing activities:
Net increase in advances from subsidiaries			4.6	1.8	206.5
Proceeds from issuance of common stock, net of stock issuance costs			0.0	0.0	0.1
Purchase and retirement of common stock			(1.2)	(34.0)	(198.9)
Dividends paid to common stockholders			(195.9)	(195.1)	(182.1)
Net cash used in financing activities			(192.5)	(227.3)	(174.4)
Net increase (decrease) in cash and cash equivalents			(2.3)	120.9	(26.3)
Cash and cash equivalents at beginning of period			275.7	154.8	181.1
Cash and cash equivalents at end of period			$ 273.4	$ 275.7	$ 154.8
Great Western Bank
Cash flows from financing activities:
Stock Issued		$ 1,722.5