Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions	Apr. 08, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liabilities assumed:
Goodwill		$ 621.6	$ 621.6	$ 621.6	$ 621.6	$ 546.7
Idaho Independent Bank, As Recorded By Idaho Independent Bank
Assets acquired:
Mortgage loans held for sale	$ 0.5
Allowance for loan loss	(6.3)
Premises and equipment	16.5
Other real estate owned (“OREO”)	0.4
Company owned life insurance	15.2
Core deposit intangible assets	0.0
Deferred tax assets, net	3.2
Other assets	8.6
Total assets acquired	719.1
Liabilities assumed:
Deposits	596.5
Accounts payable and accrued expense	15.2
Other borrowed funds	4.0
Securities sold under repurchase agreements	30.4
Total liabilities assumed	646.1
Net assets acquired	73.0
Idaho Independent Bank, Fair Value Adjustments
Assets acquired:
Cash and cash equivalents	0.0
Investment securities	0.5
Loans held for investment	(9.8)
Mortgage loans held for sale	0.0
Allowance for loan loss	6.3
Premises and equipment	4.8
Other real estate owned (“OREO”)	2.0
Company owned life insurance	0.0
Core deposit intangible assets	13.6
Deferred tax assets, net	(2.6)
Other assets	(0.7)
Total assets acquired	14.1
Liabilities assumed:
Deposits	0.1
Accounts payable and accrued expense	2.6
Other borrowed funds	0.1
Securities sold under repurchase agreements	0.0
Total liabilities assumed	2.8
Net assets acquired	11.3
Idaho Independent Bank
Assets acquired:
Cash and cash equivalents	270.7
Investment securities	63.2
Loans held for investment	337.8
Mortgage loans held for sale	0.5
Allowance for loan loss	0.0
Premises and equipment	21.3
Other real estate owned (“OREO”)	2.4
Company owned life insurance	15.2
Core deposit intangible assets	13.6
Deferred tax assets, net	0.6
Other assets	7.9
Total assets acquired	733.2
Liabilities assumed:
Deposits	596.6
Accounts payable and accrued expense	17.8
Other borrowed funds	4.1
Securities sold under repurchase agreements	30.4
Total liabilities assumed	648.9
Net assets acquired	84.3
Class A common stock	157.3
Total consideration paid	157.3
Goodwill	73.0
Northwest Bank Acquisition, As Recorded By Inland Northwest Bank
Assets acquired:
Cash and cash equivalents	270.7
Investment securities	62.7
Loans held for investment	$ 347.6