Segment Information - Results of Operations and Selected Financial Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net interest income (expense)	$ 65,673	$ 57,233	$ 193,588	$ 168,440
Noninterest income	32,801	29,169	93,348	85,237
Income before taxes	41,918	32,526	118,695	99,347
Total Assets	7,602,438		7,602,438		$ 7,253,156
Operating Segments [Member] | Metropolitan Banks [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	21,313	18,617	62,701	54,969
Noninterest income	4,266	3,981	12,391	11,748
Income before taxes	16,538	11,499	46,157	36,357
Total Assets	2,724,502		2,724,502		2,552,024
Operating Segments [Member] | Community Banks [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	42,773	37,310	126,223	109,480
Noninterest income	15,404	13,884	44,189	40,821
Income before taxes	27,515	23,810	81,805	71,038
Total Assets	4,841,853		4,841,853		4,544,196
Operating Segments [Member] | Other Financial Services [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	1,241	1,332	4,074	4,525
Noninterest income	10,264	9,217	28,784	24,983
Income before taxes	4,440	3,762	13,511	10,526
Total Assets	80,411		80,411		117,332
Operating Segments [Member] | Executive, Operations & Support [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	346	(26)	590	(534)
Noninterest income	36,503	24,470	102,306	74,586
Income before taxes	26,344	15,137	69,881	47,368
Total Assets	945,221		945,221		885,590
Eliminations [Member]
Segment Reporting Information [Line Items]
Noninterest income	(33,636)	(22,383)	(94,322)	(66,901)
Income before taxes	(32,919)	$ (21,682)	(92,659)	$ (65,942)
Total Assets	$ (989,549)		$ (989,549)		$ (845,986)