Goodwill, Core Deposit Premium and Other Intangible Assets (Mortgage and commercial loans servicing narrative) (Details)		3 Months Ended		9 Months Ended		12 Months Ended
	May 18, 2018 USD ($) subsidiary	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Valuation servicing rights in tranches [Line Items]
Mortgage loans serviced for others		$ 4,160,000,000		$ 4,160,000,000		$ 3,560,000,000
Custodial escrow balances maintained		43,200,000		43,200,000		17,300,000
Number of subsidiaries | subsidiary	2
Total net loans receivable held to maturity			$ 773,900,000
Amount derecognized						6,900,000
Loans serviced		32,039,000		32,039,000		25,857,000
Valuation allowance				0
Mortgage Servicing Rights
Valuation servicing rights in tranches [Line Items]
Servicing asset at amortized value		47,581,000	35,002,000	47,581,000	$ 35,002,000	37,100,000
Book value of loans		30,171,000	23,742,000	30,171,000	23,742,000	$ 23,248,000	$ 32,088,000
Amount derecognized				$ 0	6,940,000
Prepayment rate				8.46%		9.73%
Discount rate				9.19%		9.06%
Fees collected for servicing of mortgage loans		2,600,000	2,900,000	$ 7,300,000	9,300,000
Mortgage Servicing Rights 15-year Tranche
Valuation servicing rights in tranches [Line Items]
Servicing asset, agreement term	15 years
Mortgage Servicing Rights 30-year Tranche
Valuation servicing rights in tranches [Line Items]
Servicing asset, agreement term	30 years
Commercial Servicing Rights
Valuation servicing rights in tranches [Line Items]
Servicing asset at amortized value		2,529,000	3,458,000	2,529,000	3,458,000	$ 3,200,000
Book value of loans		1,868,000	2,857,000	1,868,000	2,857,000	2,609,000	$ 3,690,000
Loans serviced		114,500,000		$ 114,500,000		$ 139,900,000
Commercial Servicing Rights | Minimum
Valuation servicing rights in tranches [Line Items]
Prepayment rate				9.64%		7.27%
Discount rate				13.28%		13.04%
Commercial Servicing Rights | Maximum
Valuation servicing rights in tranches [Line Items]
Prepayment rate				11.94%		8.88%
Discount rate				16.55%		15.49%
Less Than 20 Years
Valuation servicing rights in tranches [Line Items]
Servicing asset at amortized value		607,000		$ 607,000		$ 740,000
Book value of loans		315,000		$ 315,000		537,000
Servicing asset, agreement term				20 years
Fees collected for servicing of mortgage loans		401,000	$ 394,000	$ 1,200,000	$ 1,200,000
Valuation allowance				$ 0		$ 0
Amortization period				20 years		20 years
More Than 20 Years
Valuation servicing rights in tranches [Line Items]
Servicing asset at amortized value		1,922,000		$ 1,922,000		$ 2,481,000
Book value of loans		$ 1,553,000		$ 1,553,000		2,084,000
Servicing asset, agreement term				20 years
Valuation allowance				$ 0		$ 12,000
Amortization period				20 years		20 years
First Bank Lubbock Bancshares, Inc. | Mortgage Servicing Rights
Valuation servicing rights in tranches [Line Items]
Servicing asset at amortized value	$ 7,000,000
Book value of loans	$ 640,700,000
Measurement Input, Cap Rate | Mortgage Servicing Rights | Minimum
Valuation servicing rights in tranches [Line Items]
Average capitalization rate (percent)		0.0089		0.0089		0.0094
Measurement Input, Cap Rate | Mortgage Servicing Rights | Maximum
Valuation servicing rights in tranches [Line Items]
Average capitalization rate (percent)		0.0125		0.0125		0.0150
Measurement Input, Cap Rate | Commercial Servicing Rights | Minimum
Valuation servicing rights in tranches [Line Items]
Average capitalization rate (percent)		0.0310		0.0310		0.0310
Measurement Input, Cap Rate | Commercial Servicing Rights | Maximum
Valuation servicing rights in tranches [Line Items]
Average capitalization rate (percent)		0.0445		0.0445		0.0445