Segment Information - Results of Operations and Selected Financial Information (Detail) - 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]
Net interest income (expense)	$ 73,939	$ 72,287	$ 68,792	$ 66,903	$ 66,888	$ 65,673	$ 64,880	$ 63,035	$ 281,921	$ 260,476	$ 227,139
Provision for loan losses	1,412	2,758	2,433	1,684	1,516	747	1,225	314	8,287	3,802	8,512
Noninterest income	35,524	$ 35,627	$ 34,077	$ 32,001	31,851	$ 32,801	$ 30,437	$ 30,110	137,229	125,199	118,070
Depreciation and amortization									15,989	13,546	11,791
Other expenses									225,312	208,575	188,601
Income before taxes									169,562	159,752	136,305
Total assets	8,565,758				7,574,258				8,565,758	7,574,258	7,253,156
Capital expenditures									27,054	51,863	18,007
Operating Segments [Member] | Metropolitan Banks [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)									86,511	84,043	74,274
Provision for loan losses									2,143	79	2,457
Noninterest income									18,608	16,625	15,698
Depreciation and amortization									2,463	2,364	2,192
Other expenses									37,549	36,873	35,488
Income before taxes									62,964	61,352	49,835
Total assets	2,806,021				2,743,876				2,806,021	2,743,876	2,552,024
Capital expenditures									4,349	3,003	3,606
Operating Segments [Member] | Community Banks [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)									177,330	170,096	147,731
Provision for loan losses									5,606	4,178	4,442
Noninterest income									64,485	60,004	54,353
Depreciation and amortization									10,218	9,092	7,790
Other expenses									109,044	106,674	93,619
Income before taxes									116,947	110,156	96,233
Total assets	4,998,247				4,892,946				4,998,247	4,892,946	4,544,196
Capital expenditures									11,219	21,393	12,334
Operating Segments [Member] | Pegasus Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)									8,657
Provision for loan losses									140
Noninterest income									483
Depreciation and amortization									274
Other expenses									4,939
Income before taxes									3,787
Total assets	738,351								738,351
Capital expenditures									83
Operating Segments [Member] | Other Financial Services [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)									5,763	5,341	5,770
Provision for loan losses									(69)	(466)	1,767
Noninterest income									40,270	37,743	37,842
Depreciation and amortization									603	588	584
Other expenses									25,408	24,895	24,474
Income before taxes									20,091	18,067	16,787
Total assets	102,442				84,706				102,442	84,706	117,332
Capital expenditures									1,220	1,035	64
Operating Segments [Member] | Executive, Operations & Support [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)									3,660	996	(636)
Provision for loan losses									467	11	(154)
Noninterest income									152,320	137,364	97,368
Depreciation and amortization									2,431	1,502	1,225
Other expenses									50,941	42,319	36,531
Income before taxes									102,141	94,528	59,130
Total assets	950,920				861,782				950,920	861,782	885,590
Capital expenditures									10,183	26,432	2,003
Eliminations [Member]
Segment Reporting Information [Line Items]
Noninterest income									(138,937)	(126,537)	(87,191)
Other expenses									(2,569)	(2,186)	(1,511)
Income before taxes									(136,368)	(124,351)	(85,680)
Total assets	$ (1,030,223)				$ (1,009,052)				$ (1,030,223)	$ (1,009,052)	$ (845,986)