Loans Acquired - Carrying Value of all Acquired Impaired Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	$ 11,858,195		$ 10,779,685
Allowance for loan losses	56,703	$ 53,776	42,086	$ 43,108	$ 41,770	$ 37,240
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans			26,962
Loans Acquired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	3,734,921		5,074,076
Allowance for loan losses	1,345	$ 2,044	418	$ 391		$ 954
Loans Acquired | Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	21,278		51,467
Loans Acquired | Real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	3,114,950		3,818,037
Loans Acquired | Real estate | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	2,040,189		2,387,777
Loans Acquired | Real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	467,895		637,032
Loans Acquired | Real estate | Single family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	606,866		793,228
Loans Acquired | Real estate | Other commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	2,040,189		2,387,777
Loans Acquired | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	598,693		1,062,163
Loans Acquired | Commercial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	596,641		995,587
Loans Acquired | Commercial | Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	2,052		66,576
Loans Acquired | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	$ 0		$ 142,409