Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 160.2	$ 181.0	$ 160.2	$ 106.5
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation expense		8.0	5.6	3.9
Net cash provided by operating activities		127.3	219.0	154.6
Cash flows from investing activities:
Acquisition of intangible assets		0.0	0.0	(28.0)
Acquisition of banks and bank holding companies, net of cash and cash equivalents acquired		298.4	(28.1)	91.8
Net cash used in investing activities		(16.8)	181.1	(130.6)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of stock issuance costs		1.0	1.8	2.4
Purchase and retirement of common stock		(2.5)	(1.0)	(1.3)
Dividends paid to common stockholders		(79.2)	(64.1)	(48.6)
Net cash used in financing activities		144.3	25.2	(47.1)
Net change in cash and cash equivalents				(23.1)
Cash and cash equivalents at beginning of period	758.9		758.9	782.0
Cash and cash equivalents at end of period				758.9
Parent Company
Cash flows from operating activities:
Net income		181.0	160.2	106.5
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries		(35.6)	(37.7)	11.0
Stock-based compensation expense		8.0	5.6	3.9
Other, net		8.1	16.4	14.7
Net cash provided by operating activities		161.5	144.5	136.1
Cash flows from investing activities:
Capital distributions from nonbank subsidiaries		0.0	0.0	18.0
Acquisition of intangible assets		0.0	0.0	(28.0)
Acquisition of banks and bank holding companies, net of cash and cash equivalents acquired		0.0	(14.7)	(128.3)
Investment in subsidiary		0.0	0.0	(18.0)
Net cash used in investing activities		0.0	(14.7)	(156.3)
Cash flows from financing activities:
Net (decrease) increase in advances from subsidiaries		(6.6)	(9.9)	(28.4)
Repayment of long-term debt		0.0	(26.0)	0.0
Proceeds from issuance of common stock, net of stock issuance costs		1.0	1.8	2.4
Purchase and retirement of common stock		(2.5)	(1.0)	(1.3)
Dividends paid to common stockholders		(79.2)	(64.1)	(48.6)
Net cash used in financing activities		(87.3)	(99.2)	(75.9)
Net change in cash and cash equivalents		74.2	30.6	(96.1)
Cash and cash equivalents at beginning of period	$ 42.3	72.9	42.3	138.4
Cash and cash equivalents at end of period		147.1	72.9	42.3
Community 1st Bank [Member]
Cash flows from financing activities:
Stock Issued		$ 176.1
Inland Northwest Bank [Member]
Cash flows from financing activities:
Stock Issued			$ 173.3
BOTC Acquisition
Cash flows from financing activities:
Stock Issued				$ 386.0