Goodwill and Core Deposit Intangibles - Schedule of Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Property, Plant and Equipment, Net	$ 245.2	$ 241.9
Goodwill [Roll Forward]
Net carrying value at beginning of period	444.7	212.8
Acquisitions and measurement period adjustments	102.0	231.9
Net carrying value at end of period	546.7	$ 444.7
Inland Northwest Bank [Member]
Goodwill [Line Items]
Change in goodwill	$ 0.3