Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions		3 Months Ended
	May 30, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Goodwill		$ 445.6	$ 212.8	$ 444.7	$ 212.8
Net increase to goodwill		0.9	$ 0.0
Cascade Bank Acquisition, as recorded by Cascade Bank
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
Business Acquisition [Line Items]
Cash and cash equivalents	0.0
Investment securities	4.9
Loans held for investment	(31.7)
Mortgage loans held for sale	0.0
Allowance for loan loss	24.0
Premises and equipment	0.1
Other real estate owned (OREO)	0.0
Core deposit intangible assets	48.0
Deferred tax assets, net	(20.9)
Other assets	2.1
Total assets acquired	26.5
Deposits	(0.9)
Accounts Payable and Accrued Expense	1.9
Total liabilities assumed	1.0
Net assets acquired	25.5
Cascade Bank Acquisition, as Recorded by the Company
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
Goodwill	$ 232.8
Net increase to goodwill		$ 0.9