Mergers and Acquisitions - Narrative (Details) $ in Thousands	Sep. 01, 2018 USD ($) branch shares	Jul. 01, 2017 USD ($) branch shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Goodwill resulting from merger			$ 932,928	$ 611,046	$ 470,534
Brand Group Holdings, Inc.
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	$ 349,416
Goodwill resulting from merger	321,882
Core deposit intangible	$ 27,534
Weighted average useful life (in years)	10 years
Metropolitan Bancgroup, Inc.
Business Acquisition [Line Items]
Transaction value		$ 219,461
Shares issued to common shareholders (in shares) | shares		4,883,182
Cash settlement for stock options		$ 4,764
Voting interest acquired (percent)		100.00%
Number of locations acquired | branch		8
Intangible assets, including goodwill		$ 147,478
Goodwill resulting from merger		140,512
Core deposit intangible		$ 6,966
Weighted average useful life (in years)		10 years