Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income	$ 161.2	$ 181.0	$ 160.2
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation expense	7.5	8.0	5.6
Net cash provided by operating activities	268.3	127.3	219.0
Cash flows from investing activities:
Acquisition of banks and bank holding companies, net of cash and cash equivalents received	0.0	(298.4)	(28.1)
Net cash used in investing activities	(1,869.8)	(16.8)	(181.1)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of stock issuance costs	1.1	1.0	1.8
Purchase and retirement of common stock	(116.8)	(2.5)	(1.0)
Dividends paid to common stockholders	(128.6)	(79.2)	(64.1)
Net cash used in financing activities	2,801.5	144.3	25.2
Net increase (decrease) in cash and cash equivalents	1,200.0	254.8	63.1
Cash and cash equivalents at beginning of period	1,076.8	822.0	758.9
Cash and cash equivalents at end of period	2,276.8	1,076.8	822.0
Parent Company
Cash flows from operating activities:
Net income	161.2	181.0	160.2
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	(50.0)	(35.6)	(37.7)
Stock-based compensation expense	7.5	8.0	5.6
Other, net	(13.6)	8.1	16.4
Net cash provided by operating activities	105.1	161.5	144.5
Cash flows from investing activities:
Acquisition of banks and bank holding companies, net of cash and cash equivalents received	0.0	0.0	(14.7)
Net cash used in investing activities	0.0	0.0	(14.7)
Cash flows from financing activities:
Net (decrease) increase in advances from subsidiaries	16.7	(6.6)	(9.9)
Proceeds from issuance of long-term debt	98.6	0.0	0.0
Repayment of long-term debt	0.0	0.0	(26.0)
Proceeds from issuance of common stock, net of stock issuance costs	1.1	1.0	1.8
Purchase and retirement of common stock	(116.8)	(2.5)	(1.0)
Dividends paid to common stockholders	(128.6)	(79.2)	(64.1)
Net cash used in financing activities	(129.0)	(87.3)	(99.2)
Net increase (decrease) in cash and cash equivalents	(23.9)	74.2	30.6
Cash and cash equivalents at beginning of period	147.1	72.9	42.3
Cash and cash equivalents at end of period	123.2	147.1	72.9
Community 1st Bank [Member]
Cash flows from financing activities:
Stock Issued	$ 0.0
Inland Northwest Bank [Member]
Cash flows from financing activities:
Stock Issued		$ 176.1
BOTC Acquisition
Cash flows from financing activities:
Stock Issued			$ 173.3