The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Goodwill and Other Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Oct. 19, 2017	Oct. 18, 2017	May 15, 2017	May 14, 2017	Sep. 09, 2016
Goodwill [Line Items]
Goodwill			$ 845,687	$ 842,651
Goodwill, written off related to sale of business unit		$ 4,100
Finite-lived intangible assets, period decrease	$ 3,800	$ 1,300
Amortization of intangibles			$ 11,009	7,668	$ 5,945
Minimum
Goodwill [Line Items]
Finite-lived intangible asset, useful life			10 years
Maximum
Goodwill [Line Items]
Finite-lived intangible asset, useful life			15 years
Core deposit intangible
Goodwill [Line Items]
Amortization of intangibles			$ 9,500	6,000	4,400
Core deposit intangible \| Minimum
Goodwill [Line Items]
Finite-lived intangible asset, useful life			10 years
Core deposit intangible \| Maximum
Goodwill [Line Items]
Finite-lived intangible asset, useful life			15 years
Purchased Credit Card Relationships
Goodwill [Line Items]
Amortization of intangibles				310	414
Books of Business Intangible
Goodwill [Line Items]
Amortization of intangibles			$ 1,100	1,400	1,400
OKSB Merger
Goodwill [Line Items]
Goodwill						$ 229,144	$ 13,545
Finite-lived intangible assets acquired				3,800
OKSB Merger \| Core deposit intangible
Goodwill [Line Items]
Finite-lived intangible assets acquired				42,100
First Texas BHC, Inc.
Goodwill [Line Items]
Goodwill						$ 240,800
First Texas BHC, Inc. \| Core deposit intangible
Goodwill [Line Items]
Finite-lived intangible assets acquired				7,300
Hardeman County Investment Company, Inc.
Goodwill [Line Items]
Goodwill								$ 29,371	$ 11,485
Finite-lived intangible assets acquired				830
Hardeman County Investment Company, Inc. \| Core deposit intangible
Goodwill [Line Items]
Finite-lived intangible assets acquired				$ 7,800
Citizens National Bank
Goodwill [Line Items]
Goodwill					21,500					$ 23,376
Finite-lived intangible assets acquired					591
Citizens National Bank \| Core deposit intangible
Goodwill [Line Items]
Finite-lived intangible assets acquired					$ 5,100