Acquisitions - Schedules of Loans Acquired (Details) - USD ($) $ in Millions	1 Months Ended	2 Months Ended
	Jun. 30, 2017	Sep. 30, 2018	Aug. 16, 2018	May 30, 2017
Business Acquisition [Line Items]
Contractually required principal and interest payments				$ 49.7
Contractual cash flows not expected to be collected (“non-accretable discount”)				24.7
Cash flows expected to be collected				25.0
Interest component of cash flows expected to be collected (“accretable discount”)				1.9
Fair value of acquired credit-impaired loans				23.1
Contractually required principal and interest payments				2,098.1
Contractual cash flows not expected to be collected				23.3
Fair value at acquisition				$ 2,066.5
Cascade Bank
Business Acquisition [Line Items]
Revenue reported by Cascade	$ 12.9
Net income reported by Cascade	$ 3.0
Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Contractually required principal and interest payments			$ 27.5
Contractual cash flows not expected to be collected (“non-accretable discount”)			4.4
Cash flows expected to be collected			23.1
Interest component of cash flows expected to be collected (“accretable discount”)			3.2
Fair value of acquired credit-impaired loans			19.9
Contractually required principal and interest payments			894.8
Contractual cash flows not expected to be collected			26.1
Fair value at acquisition			$ 693.2
Revenue reported by Cascade		$ 5.9
Net income reported by Cascade		$ 2.1