Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions		12 Months Ended
	Feb. 01, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Cash flows from operating activities:
Net income		$ 202.2	$ 192.1	$ 161.2
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation expense		9.6	8.9	7.5
Net cash provided by operating activities		534.4	282.3	268.3
Cash flows from investing activities:
Net cash used in investing activities		(949.0)	(2,120.0)	(1,869.8)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of stock issuance costs		0.1	0.4	1.1
Purchase and retirement of common stock		(199.0)	(5.4)	(116.8)
Dividends paid to common stockholders		(182.1)	(101.6)	(128.6)
Net cash used in financing activities		(1,059.7)	1,905.7	2,801.5
Net (decrease) increase in cash and cash equivalents		(1,474.3)	68.0	1,200.0
Cash and cash equivalents at beginning of period		2,344.8	2,276.8	1,076.8
Cash and cash equivalents at end of period		870.5	2,344.8	2,276.8
Parent Company
Cash flows from operating activities:
Net income		202.2	192.1	161.2
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries		88.3	(57.0)	(50.0)
Stock-based compensation expense		9.6	8.9	7.5
Other, net		(151.2)	(3.2)	(13.6)
Net cash provided by operating activities		148.9	140.8	105.1
Cash flows from investing activities:
Acquisition of bank and bank holding company, net of cash and cash equivalents received		(0.8)	0.0	0.0
Net cash used in investing activities		(0.8)	0.0	0.0
Cash flows from financing activities:
Net increase in advances from subsidiaries		206.5	23.7	16.7
Proceeds from issuance of long-term debt		0.0	0.0	98.6
Proceeds from issuance of common stock, net of stock issuance costs		0.1	0.4	1.1
Purchase and retirement of common stock		(198.9)	(5.4)	(116.8)
Dividends paid to common stockholders		(182.1)	(101.6)	(128.6)
Net cash used in financing activities		(174.4)	(82.9)	(129.0)
Net (decrease) increase in cash and cash equivalents		(26.3)	57.9	(23.9)
Cash and cash equivalents at beginning of period		181.1	123.2	147.1
Cash and cash equivalents at end of period		$ 154.8	$ 181.1	$ 123.2
Great Western Bank
Cash flows from financing activities:
Stock Issued	$ 1,722.5