Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions			3 Months Ended		12 Months Ended
		Aug. 16, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs			$ 2.3	$ 2.3	$ 2.3
Goodwill			546.3	445.6	$ 546.7	$ 444.7
Net increase to goodwill			(0.4)	$ 0.9
Northwest Bank Acquisition, As Recorded By Inland Northwest Bank [Member]
Business Acquisition [Line Items]
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
Deposits		696.1
Accounts Payable and Accrued Expense		8.1
Long term debt		13.0
Trust preferred securities		5.2
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability		(1.2)
Total liabilities assumed		721.2
Net assets acquired		79.5
Northwest Bank Acquisition, Fair Value Adjustments [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		0.0
Investment securities		0.0
Loans held for investment	[1]	(14.8)
Allowance for loan loss	[2]	8.0
Premises and equipment		0.0
Other real estate owned (“OREO”)		0.3
Core deposit intangible assets	[3]	13.3
Other assets	[4]	(10.0)
Total assets acquired		(3.2)
Deposits	[5]	0.2
Accounts Payable and Accrued Expense	[6]	(0.4)
Long term debt		0.1
Trust preferred securities		(0.8)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability	[7]	1.6
Total liabilities assumed		0.7
Net assets acquired		(3.9)
Northwest Bank Acquisition, as Recorded by the Company [Member]
Business Acquisition [Line Items]
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
Deposits		696.3
Accounts Payable and Accrued Expense		7.7
Long term debt		13.1
Trust preferred securities		4.4
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability		0.4
Total liabilities assumed		721.9
Net assets acquired		75.6
Cash		3.0
Class A common stock		173.3
Inland Northwest Bank
Business Acquisition [Line Items]
Total consideration paid		176.3
Goodwill		$ 100.7
Cascade Bank Acquisition, as Recorded by the Company [Member]
Business Acquisition [Line Items]
Net increase to goodwill			$ 0.9

[1] (1) Write down of the book value of loans to their estimated fair values. The fair value of the loans was estimated using cash flow projections based on the remaining maturity and repricing terms, adjusted for estimated future credit losses and prepayments and discounted to present value using a risk-adjusted market rate for similar loans. The fair value of collateral dependent loans acquired with deteriorated credit quality was estimated based on the Company’s analysis of the fair value of each loan’s underlying collateral, discounted using market-derived rates of return with consideration given to the period of time and costs associated with foreclosure and disposition of the collateral.

[2] (2) Adjustment to remove the Northwest allowance for loan losses at acquisition date, as the credit risk is included in the fair value adjustment for loans receivable described in (1) above.

[3] (3) Write down of the book value of premises and equipment to their estimated fair values on the date of acquisition based upon broker’s opinion of value.

[4] (4) Adjustment represents the value of the core deposit base assumed in the acquisition based upon valuation from an independent accounting and advisory firm.

[5] (6) Adjustment consists of reductions to the fair value of other items, including the removal of Northwest previously recorded goodwill.

[6] (7) Increase in book value of time deposits to their estimated fair values based upon interest rates of similar time deposits with similar terms on the date of acquisition based upon valuation from an independent accounting and advisory firm.

[7] (9) Write down of the book value of debt to the estimated fair values on the date of acquisition based upon favorable interest rates in the market.