Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions	Apr. 08, 2019	Aug. 16, 2018	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Liabilities assumed:
Goodwill			$ 623.5	$ 546.7	$ 444.7
Idaho Independent Bank, As Recorded By Idaho Independent Bank
Assets acquired:
Cash and cash equivalents	$ 270.7
Investment securities	62.7
Loans held for investment	347.6
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	3.8
Other real estate owned (“OREO”)	2.0
Company owned life insurance	0.0
Core deposit intangible assets	13.6
Deferred tax assets, net	(3.0)
Other assets	(0.8)
Total assets acquired	12.6
Liabilities assumed:
Deposits	0.1
Accounts payable and accrued expense	2.8
Other borrowed funds	0.1
Securities sold under repurchase agreements	0.0
Total liabilities assumed	3.0
Net assets acquired	9.6
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	20.3
Other real estate owned (“OREO”)	2.4
Company owned life insurance	15.2
Core deposit intangible assets	13.6
Deferred tax assets, net	0.2
Other assets	7.8
Total assets acquired	731.7
Liabilities assumed:
Deposits	596.6
Accounts payable and accrued expense	18.0
Other borrowed funds	4.1
Securities sold under repurchase agreements	30.4
Total liabilities assumed	649.1
Net assets acquired	82.6
Class A common stock	157.3
Total consideration paid	157.3
Goodwill	$ 74.7
Northwest Bank Acquisition, As Recorded By Inland Northwest Bank
Assets acquired:
Cash and cash equivalents		$ 31.2
Investment securities		3.1
Loans held for investment		727.9
Allowance for loan loss		(8.0)
Premises and equipment		14.5
Other real estate owned (“OREO”)		0.3
Core deposit intangible assets		2.4
Other assets		29.3
Total assets acquired		800.7
Liabilities assumed:
Deposits		696.1
Other borrowed funds		8.1
Long term debt		13.0
Trust preferred securities		5.2
Deferred tax liability, net		(1.2)
Total liabilities assumed		721.2
Net assets acquired		79.5
Northwest Bank Acquisition, Fair Value Adjustments
Assets acquired:
Cash and cash equivalents		0.0
Investment securities		0.0
Loans held for investment		(14.8)
Allowance for loan loss		8.0
Premises and equipment		0.0
Other real estate owned (“OREO”)		0.3
Core deposit intangible assets		13.3
Other assets		(10.0)
Total assets acquired		(3.2)
Liabilities assumed:
Deposits		0.2
Other borrowed funds		(0.4)
Long term debt		0.1
Trust preferred securities		(0.8)
Deferred tax liability, net		1.6
Total liabilities assumed		0.7
Net assets acquired		(3.9)
Inland Northwest Bank
Assets acquired:
Cash and cash equivalents		31.2
Investment securities		3.1
Loans held for investment		713.1
Allowance for loan loss		0.0
Premises and equipment		14.5
Other real estate owned (“OREO”)		0.6
Core deposit intangible assets		15.7
Other assets		19.3
Total assets acquired		797.5
Liabilities assumed:
Deposits		696.3
Other borrowed funds		7.7
Long term debt		13.1
Trust preferred securities		4.4
Deferred tax liability, net		0.4
Total liabilities assumed		721.9
Net assets acquired		75.6
Cash		3.0
Class A common stock		173.3
Total consideration paid		176.3
Goodwill		$ 100.7