Segment Information - Results of Operations and Selected Financial Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net interest income (expense)	$ 66,903	$ 63,035
Noninterest income	32,001	30,110
Income before taxes	41,014	36,941
Total Assets	7,709,000		$ 7,574,258
Operating Segments [Member] | Metropolitan Banks [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	21,353	20,304
Noninterest income	4,209	3,941
Income before taxes	15,369	14,736
Total Assets	2,763,523		2,743,876
Operating Segments [Member] | Community Banks [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	43,407	41,382
Noninterest income	14,885	13,953
Income before taxes	28,045	26,348
Total Assets	4,995,379		4,892,946
Operating Segments [Member] | Other Financial Services [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	1,171	1,475
Noninterest income	9,873	9,624
Income before taxes	4,499	5,043
Total Assets	69,430		84,706
Operating Segments [Member] | Executive, Operations & Support [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	972	(126)
Noninterest income	35,521	32,194
Income before taxes	25,023	19,915
Total Assets	912,166		861,782
Eliminations [Member]
Segment Reporting Information [Line Items]
Noninterest income	(32,487)	(29,602)
Income before taxes	(31,922)	$ (29,101)
Total Assets	$ (1,031,498)		$ (1,009,052)