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, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities:
Net income			$ 257.5	$ 202.2	$ 192.1
Adjustments to reconcile net income to cash provided by operating activities:
Net cash provided by operating activities			428.0	534.4	282.3
Cash flows from investing activities:
Net cash used in investing activities			1,253.4	(949.0)	(2,120.0)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of stock issuance costs			0.0	0.1	0.4
Purchase and retirement of common stock	$ (32.1)		(34.0)	(199.0)	(5.4)
Dividends paid to common stockholders			(195.1)	(182.1)	(101.6)
Net cash used in financing activities			(1,973.9)	(1,059.7)	1,905.7
Net (decrease) increase in cash and cash equivalents			(292.5)	(1,474.3)	68.0
Cash and cash equivalents at beginning of period			870.5	2,344.8	2,276.8
Cash and cash equivalents at end of period			578.0	870.5	2,344.8
Parent Company
Cash flows from operating activities:
Net income			257.5	202.2	192.1
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries			83.4	88.3	(57.0)
Stock-based compensation expense			4.6	9.6	8.9
Other, net			2.7	(151.2)	(3.2)
Net cash provided by operating activities			348.2	148.9	140.8
Cash flows from investing activities:
Acquisition of bank and bank holding company, net of cash and cash equivalents received			0.0	(0.8)	0.0
Net cash used in investing activities			0.0	(0.8)	0.0
Cash flows from financing activities:
Net increase in advances from subsidiaries			1.8	206.5	23.7
Proceeds from issuance of common stock, net of stock issuance costs			0.0	0.1	0.4
Purchase and retirement of common stock			(34.0)	(198.9)	(5.4)
Dividends paid to common stockholders			(195.1)	(182.1)	(101.6)
Net cash used in financing activities			(227.3)	(174.4)	(82.9)
Net (decrease) increase in cash and cash equivalents			120.9	(26.3)	57.9
Cash and cash equivalents at beginning of period			154.8	181.1	123.2
Cash and cash equivalents at end of period			$ 275.7	$ 154.8	$ 181.1
Great Western Bank
Cash flows from financing activities:
Stock Issued		$ 1,722.5