Segment Reporting (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 77,009	$ 77,101	$ 75,851	$ 67,776	$ 69,630	$ 67,676	$ 64,742	$ 64,850	$ 297,737	$ 266,898	$ 243,759
Provision for loan losses	(213)	1,967	1,919	2,498	3,359	2,940	1,386	260	6,171	7,945	8,557
Other Income (Loss) and Security Gains									97,193	101,101	86,429
Noninterest Expense									263,988	228,755	203,162
Income before income taxes	30,215	37,532	26,548	30,476	30,566	29,484	34,064	37,185	124,771	131,299	118,469
Income taxes									22,071	20,912	34,227
Net Income (Loss) Attributable to Parent	23,936	$ 31,146	$ 22,163	$ 25,455	26,261	$ 24,762	$ 28,241	$ 31,123	102,700	110,387	84,242
Assets	8,558,377				7,804,308				8,558,377	7,804,308	7,537,620
Total loans	6,501,404				5,692,132				6,501,404	5,692,132	5,372,483
Total deposits	7,052,612				6,260,860				7,052,612	6,260,860	5,817,326
Depreciation, Amortization and Accretion, Net									9,112	8,585	8,644
Other Noninterest Expense									254,876	220,170	194,518
Segment One [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									293,130	258,547	235,243
Provision for loan losses									8,356	7,569	9,898
Other Income (Loss) and Security Gains									92,392	88,981	82,742
Noninterest Expense									237,433	206,843	185,891
Income before income taxes									139,733	133,116	122,196
Income taxes									26,133	23,644	34,881
Net Income (Loss) Attributable to Parent									113,600	109,472	87,315
Assets	8,521,537				7,753,848				8,521,537	7,753,848	7,467,851
Total loans	6,481,644				5,671,173				6,481,644	5,671,173	5,339,255
Total deposits	7,125,111				6,334,796				7,125,111	6,334,796	5,896,676
GFSC [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									5,013	5,048	5,839
Provision for loan losses									754	1,328	1,917
Other Income (Loss) and Security Gains									170	187	103
Noninterest Expense									3,478	3,245	3,099
Income before income taxes									951	662	926
Income taxes									189	141	666
Net Income (Loss) Attributable to Parent									762	521	260
Assets	27,593				31,388				27,593	31,388	32,077
Total loans	28,143				32,664				28,143	32,664	33,385
Total deposits	3,919				4,142				3,919	4,142	3,449
Other Segments [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									(406)	3,303	2,677
Provision for loan losses									(2,939)	(952)	(3,258)
Other Income (Loss) and Security Gains									4,631	11,933	3,584
Noninterest Expense									23,077	18,667	14,172
Income before income taxes									(15,913)	(2,479)	(4,653)
Income taxes									(4,251)	(2,873)	(1,320)
Net Income (Loss) Attributable to Parent									(11,662)	394	(3,333)
Assets	9,247				19,072				9,247	19,072	37,692
Total loans	(8,383)				(11,705)				(8,383)	(11,705)	(157)
Total deposits	(76,418)				(78,078)				(76,418)	(78,078)	(82,799)
Totals For Reportable Segments [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									298,143	263,595	241,082
Income taxes									26,322	23,785	35,547
Assets	8,549,130				7,785,236				8,549,130	7,785,236	7,499,928
Total deposits	7,129,030				6,338,938				7,129,030	6,338,938	5,900,125
Depreciation, Amortization and Accretion, Net									9,112	8,585	8,644
Other Noninterest Expense									231,799	201,503	180,346
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									1,250	1,275	1,500
Income taxes									0	0	0
Assets	(19,231)				(13,482)				(19,231)	(13,482)	(11,211)
Total deposits	(76,418)				(78,078)				(76,418)	(78,078)	(82,799)
Depreciation, Amortization and Accretion, Net									0	0	0
Other Noninterest Expense									0	0	0
Parent Co Gfsc Totals Not Eliminated [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									(1,656)	2,028	1,177
Income taxes									(4,251)	(2,873)	(1,320)
Assets	28,478				32,554				28,478	32,554	48,903
Total deposits	$ 0				$ 0				0	0	0
Depreciation, Amortization and Accretion, Net									0	0	0
Other Noninterest Expense									$ 23,077	$ 18,667	$ 14,172