Mergers and Acquisitions - Narrative (Details) $ in Thousands	Sep. 01, 2018 USD ($) branch shares	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)
Business Acquisition [Line Items]
Goodwill resulting from merger		$ 932,971	$ 932,928	$ 611,046
Brand
Business Acquisition [Line Items]
Transaction value	$ 474,453
Shares issued to common shareholders (in shares) | shares	9,306,477
Cash consideration paid	$ 21,879
Cash settlement for stock options	$ 17,157
Voting interest acquired (percent)	100.00%
Number of locations acquired | branch	13
Intangible assets, including goodwill	$ 349,459
Goodwill resulting from merger	321,925
Core deposit intangible	$ 27,534
Weighted average useful life (in years)	10 years