Segment Information - Results of Operations and Selected Financial Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net interest income (expense)	$ 63,035	$ 54,768
Noninterest income	30,110	28,085
Income before taxes	36,941	33,193
Total Assets	7,615,642		$ 7,253,156
Operating Segments [Member] | Metropolitan Banks [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	20,304	18,520
Noninterest income	3,941	3,822
Income before taxes	14,736	13,074
Total Assets	2,646,469		2,552,024
Operating Segments [Member] | Community Banks [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	41,382	35,123
Noninterest income	13,953	13,277
Income before taxes	26,348	22,719
Total Assets	4,952,580		4,544,196
Operating Segments [Member] | Other Financial Services [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	1,475	1,495
Noninterest income	9,624	8,310
Income before taxes	5,043	3,645
Total Assets	84,625		117,332
Operating Segments [Member] | Executive, Operations & Support [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	(126)	(370)
Noninterest income	32,194	24,472
Income before taxes	19,915	15,511
Total Assets	838,976		885,590
Eliminations [Member]
Segment Reporting Information [Line Items]
Noninterest income	(29,602)	(21,796)
Income before taxes	(29,101)	$ (21,756)
Total Assets	$ (907,008)		$ (845,986)