Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions				3 Months Ended		9 Months Ended
		Aug. 16, 2018	May 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Class A common stock		$ 173.3
Goodwill				$ 546.4	$ 445.0	$ 546.4	$ 444.7	$ 212.8
Net increase to goodwill				$ 0.9	$ 0.0
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
Deferred tax assets, net		1.2
Other assets		29.3
Total assets acquired		801.9
Deposits		696.1
Accounts Payable and Accrued Expense		8.1
Long term debt		13.0
Trust preferred securities		5.2
Total liabilities assumed		722.4
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	[3]	(0.5)
Other real estate owned (“OREO”)		0.3
Core deposit intangible assets	[4]	13.3
Deferred tax assets, net	[5]	(1.6)
Other assets	[6]	(9.7)
Total assets acquired		(5.0)
Deposits	[7]	0.2
Accounts Payable and Accrued Expense	[8]	(0.4)
Long term debt	[9]	0.0
Trust preferred securities	[10]	(0.8)
Total liabilities assumed		(1.0)
Net assets acquired		(4.0)
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		14.0
Other real estate owned (“OREO”)		0.6
Core deposit intangible assets		15.7
Deferred tax assets, net		(0.4)
Other assets		19.6
Total assets acquired		796.9
Deposits	[9]	696.3
Accounts Payable and Accrued Expense	[9]	7.7
Long term debt	[9]	13.0
Trust preferred securities	[9]	4.4
Total liabilities assumed		721.4
Net assets acquired		75.5
Cash		3.0
Class A common stock		173.3
Inland Northwest Bank
Business Acquisition [Line Items]
Total consideration paid		176.3
Goodwill		$ 100.8
Cascade Bank Acquisition, As Recorded By Cascade Bank [Member]
Business Acquisition [Line Items]
Cash and cash equivalents			$ 246.8
Investment securities			476.7
Loans held for investment			2,111.0
Mortgage loans held for sale			10.3
Allowance for loan loss			(24.0)
Premises and equipment			46.6
Other real estate owned (“OREO”)			1.2
Core deposit intangible assets			0.0
Deferred tax assets, net			47.6
Other assets			98.6
Total assets acquired			3,014.8
Deposits			2,669.9
Accounts Payable and Accrued Expense			62.2
Total liabilities assumed			2,732.1
Net assets acquired			282.7
Cascade Bank Acquisition, Fair Value Adjustments [Member]
Business Acquisition [Line Items]
Cash and cash equivalents			0.0
Investment securities	[11]		4.9
Loans held for investment	[12]		(31.7)
Mortgage loans held for sale			0.0
Allowance for loan loss	[13]		24.0
Premises and equipment	[14]		0.1
Other real estate owned (“OREO”)			0.0
Core deposit intangible assets	[15]		48.0
Deferred tax assets, net	[16]		(20.9)
Other assets			2.1
Total assets acquired			26.5
Deposits	[9]		(0.9)
Accounts Payable and Accrued Expense	[17]		1.9
Total liabilities assumed			1.0
Net assets acquired			25.5
Bank of the Cascades
Business Acquisition [Line Items]
Cash			155.0
Total consideration paid			541.0
Goodwill			232.8
Cascade Bank Acquisition, as Recorded by the Company [Member]
Business Acquisition [Line Items]
Cash and cash equivalents			246.8
Investment securities			481.6
Loans held for investment			2,079.3
Mortgage loans held for sale			10.3
Allowance for loan loss			0.0
Premises and equipment			46.7
Other real estate owned (“OREO”)			1.2
Core deposit intangible assets			48.0
Deferred tax assets, net			26.7
Other assets			100.7
Total assets acquired			3,041.3
Deposits			2,669.0
Accounts Payable and Accrued Expense			64.1
Total liabilities assumed			2,733.1
Net assets acquired			308.2
Cash			155.0
Class A common stock			386.0
Total consideration paid			$ 541.0
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] (5) Adjustment consists of the write-off of pre-existing deferred tax assets and purchase accounting adjustments as a result of the acquisition.

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

[7] (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.

[8] (8) Decrease due to the write-off of off balance sheet reserves.

[9] Decrease 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.

[10] (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.

[11] Write up of the book value of investments to their estimated fair values on the date of acquisition based upon quotes obtained from an independent third party pricing service.

[12] Write down of the book value of loans to their estimated fair values. Shared National Credits (SNC) were recorded at quoted sales prices where available. The fair value of the remaining 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.

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

[14] Write up of the book value of premises and equipment to their estimated fair values on the date of acquisition based upon appraisals obtained from an independent third party appraiser or broker’s opinion of value.

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

[16] Adjustment consists of the write-off of pre-existing deferred tax assets as a result of the acquisition.

[17] Increase in fair value due to credit card incentive program, unfavorable leases, write-off of balance sheet reserve, and swap liability offset.