Business Combinations - American Enterprise Bankshares (Details) $ in Millions	Mar. 01, 2016 USD ($) branch shares	Jun. 30, 2016 shares	Dec. 31, 2015 shares
Business Acquisition [Line Items]
Common stock, shares issued (in shares) | shares		25,509,216	23,140,774
American Enterprise Bankshares, Inc.
Business Acquisition [Line Items]
Consideration transferred	$ 22.8
Exchange of shares received based on business acquisition	0.299
Common stock, shares issued (in shares) | shares	1,470,068
AEB and Enterprise Bank
Business Acquisition [Line Items]
Fair value of net assets assumed	$ 208.8
Recognized Identifiable Assets Acquired and Liabilities Assumed, Liquid Assets	40.7
Fair value of acquired performing loans at acquisition	147.4
Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	1.3
Customer deposits	181.8
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	7.1
Deferred Tax Assets, Valuation Allowance	5.9
Goodwill	$ 5.2
AEB and Enterprise Bank | Fidelity
Business Acquisition [Line Items]
Number of branches acquired | branch	2