Acquisitions (Acquisitions fair values) (Details) - USD ($) $ in Thousands	May 18, 2018	Jun. 30, 2018	Dec. 31, 2017
Fair value of liabilities assumed
Goodwill resulting from acquisition		$ 391,668	$ 236,615
First Bank Lubbock Bancshares, Inc.
Fair value of consideration paid:
Common stock	$ 184,454
Common stock (in shares)	3,350,664
Cash	$ 5,451
Total consideration paid	189,905
Fair value of assets acquired
Cash and due from banks	212,105
Securities:
Securities available for sale	1,788
Other securities	3,268
Loans held for sale	31,050
Loans held to maturity	681,080
Premises, furniture and equipment, net	23,271
Other real estate, net	379
Mortgage servicing rights	6,995
Core deposit intangibles and customer relationships, net	13,908
Cash surrender value on life insurance	14,997
Other assets	7,185
Total assets	996,026
Fair value of liabilities assumed
Deposits	893,827
Other borrowings	12,077
Other liabilities	21,580
Total liabilities assumed	927,484
Fair value of net assets acquired	68,542
Goodwill resulting from acquisition	$ 121,400