Loans Acquired - Carrying Value of all Acquired Impaired Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	$ 13,002,952		$ 11,723,171
Allowance for loan losses	66,590	$ 64,179	56,694	$ 56,703	$ 53,776	$ 42,086
Loans Acquired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	3,359,587		3,292,783
Allowance for loan losses	597	$ 1,112	95	$ 1,345	$ 2,044	$ 418
Loans Acquired | Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	7,431		15,658
Loans Acquired | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	3,001,463		2,844,472
Loans Acquired | Real Estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	362,000		429,605
Loans Acquired | Real Estate | Single family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	504,490		566,188
Loans Acquired | Real Estate | Other commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	2,134,973		1,848,679
Loans Acquired | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	350,693		432,653
Loans Acquired | Commercial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	349,821		430,914
Loans Acquired | Commercial | Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	$ 872		$ 1,739