Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ 160.2	$ 106.5	$ 95.7
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation expense	5.6	3.9	4.4
Tax benefits from stock-based compensation	0.0	0.0	2.1
Excess tax benefits from stock-based compensation	0.0	0.0	(1.6)
Net cash provided by operating activities	220.2	154.6	118.0
Cash flows from investing activities:
Acquisition of intangible assets	0.0	(28.0)	0.0
Acquisition of bank and bank holding company, net of cash and cash equivalents received	28.1	91.8	18.6
Net cash provided by (used in) investing activities	(182.3)	(130.6)	(143.1)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of stock issuance costs	1.8	2.4	4.7
Excess tax benefits from stock-based compensation	0.0	0.0	1.6
Purchase and retirement of common stock	(1.0)	(1.3)	(26.9)
Dividends paid to common stockholders	(64.1)	(48.6)	(39.4)
Net cash used in financing activities	25.2	(47.1)	26.6
Net change in cash and cash equivalents	63.1	(23.1)	1.5
Cash and cash equivalents at beginning of period	758.9	782.0	780.5
Cash and cash equivalents at end of period	822.0	758.9	782.0
Parent Company
Cash flows from operating activities:
Net income	160.2	106.5	95.6
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	(37.7)	11.0	32.4
Stock-based compensation expense	5.6	3.9	4.4
Tax benefits from stock-based compensation	0.0	0.0	2.1
Excess tax benefits from stock-based compensation	0.0	0.0	(1.6)
Other, net	16.4	14.7	(0.8)
Net cash provided by operating activities	144.5	136.1	132.1
Cash flows from investing activities:
Capital distributions from nonbank subsidiaries	0.0	18.0	2.0
Acquisition of intangible assets	0.0	(28.0)	0.0
Acquisition of bank and bank holding company, net of cash and cash equivalents received	(14.7)	(128.3)	0.0
Investment in subsidiary	0.0	(18.0)	0.0
Net cash provided by (used in) investing activities	(14.7)	(156.3)	2.0
Cash flows from financing activities:
Net (decrease) increase in advances from subsidiaries	(9.9)	(28.4)	9.1
Repayment of long-term debt	(26.0)	0.0	0.0
Proceeds from issuance of common stock, net of stock issuance costs	1.8	2.4	4.7
Excess tax benefits from stock-based compensation	0.0	0.0	1.6
Purchase and retirement of common stock	(1.0)	(1.3)	(26.9)
Dividends paid to common stockholders	(64.1)	(48.6)	(39.4)
Net cash used in financing activities	(99.2)	(75.9)	(50.9)
Net change in cash and cash equivalents	30.6	(96.1)	83.2
Cash and cash equivalents at beginning of period	42.3	138.4	55.2
Cash and cash equivalents at end of period	72.9	$ 42.3	$ 138.4
Inland Northwest Bank [Member]
Cash flows from financing activities:
Stock Issued	173.3
BOTC Acquisition
Cash flows from financing activities:
Stock Issued	$ 386.0