Loans Acquired - Carrying Value of all Acquired Impaired Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	$ 11,740,737	$ 11,723,171
Allowance for loan losses	60,555	56,694	$ 47,614	$ 42,086
Loans Acquired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	3,056,187	3,292,783
Allowance for loan losses	1,312	95	$ 407	$ 418
Loans Acquired | Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	11,979	15,658
Loans Acquired | Real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	2,668,901	2,844,472
Loans Acquired | Real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	404,512	429,605
Loans Acquired | Real estate | Single family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	533,917	566,188
Loans Acquired | Real estate | Other commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	1,730,472	1,848,679
Loans Acquired | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	375,307	432,653
Loans Acquired | Commercial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	374,033	430,914
Loans Acquired | Commercial | Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	$ 1,274	$ 1,739