Loans Acquired - Carrying Value of all Acquired Impaired Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	$ 11,365,895		$ 10,779,685
Allowance for loan losses	53,776	$ 47,614	42,086	$ 41,770	$ 38,300	$ 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	4,232,434		5,074,076
Allowance for loan losses	2,044	$ 407	418	$ 391	$ 435	$ 954
Loans Acquired | Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	31,651		51,467
Loans Acquired | Real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	3,433,501		3,818,037
Loans Acquired | Real estate | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	2,240,578		2,387,777
Loans Acquired | Real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	521,321		637,032
Loans Acquired | Real estate | Single family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	671,602		793,228
Loans Acquired | Real estate | Other commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	2,240,578		2,387,777
Loans Acquired | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	767,282		1,062,163
Loans Acquired | Commercial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	764,473		995,587
Loans Acquired | Commercial | Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	2,809		66,576
Loans Acquired | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired loans	$ 0		$ 142,409