Loans Acquired - Carrying Value of all Acquired Impaired Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	$ 13,127,013		$ 11,723,171
Allowance for loan losses	64,179	$ 60,555	56,694	$ 53,776	$ 47,614	$ 42,086
Loans Acquired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	3,864,516		3,292,783
Allowance for loan losses	1,112	$ 1,312	95	$ 2,044	$ 407	$ 418
Loans Acquired | Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	9,704		15,658
Loans Acquired | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	3,442,152		2,844,472
Loans Acquired | Real Estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	426,482		429,605
Loans Acquired | Real Estate | Single family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	554,534		566,188
Loans Acquired | Real Estate | Other commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	2,461,136		1,848,679
Loans Acquired | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	412,660		432,653
Loans Acquired | Commercial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	411,577		430,914
Loans Acquired | Commercial | Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	$ 1,083		$ 1,739