Acquisitions (Acquisitions Fair Values) (Details) - USD ($) $ in Thousands	May 18, 2018	Jul. 07, 2017	Dec. 31, 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 (in shares)	3,350,664
Common stock	$ 184,454
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 relationship intangibles, 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	$ 121,363
Citywide Banks of Colorado, Inc.
Fair value of consideration paid
Common stock (in shares)		3,216,161
Common stock		$ 152,607
Cash		58,636
Total consideration paid		211,243
Fair value of assets acquired
Cash and due from banks		21,341
Interest bearing deposits with the Federal Reserve and other banks and other short-term investments		74,686
Time deposits in other financial institutions		6,304
Securities:
Securities available for sale		234,390
Other securities		2,628
Loans held to maturity		985,399
Premises, furniture and equipment, net		17,206
Premises, furniture and equipment held for sale		1,350
Other real estate, net		6,916
Core deposit intangibles and customer relationship intangibles, net		16,041
Cash surrender value on life insurance		21,015
Other assets		11,263
Total assets		1,398,539
Fair value of liabilities assumed
Deposits		1,210,074
Short term borrowings		34,445
Other borrowings		21,636
Other liabilities		16,295
Total liabilities assumed		1,282,450
Fair value of net assets acquired		116,089
Goodwill resulting from acquisition		$ 95,154