Schedule of Business Segment Financial Results (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net interest income	$ 150,490	$ 140,858	$ 448,638	$ 412,567
Provision for loan losses	5,750	11,500	22,750	35,000
Noninterest income	100,885	104,306	306,699	317,529
Noninterest expense	180,385	171,821	533,479	522,570
Income before taxes	65,240	61,843	199,108	172,526
Income tax expense	7,391	12,971	28,302	36,907
Income from continuing operations	57,849	48,872	170,806	135,619
Average assets	20,750,000	20,313,000	20,704,000	20,288,000
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net interest income	94,633	89,386	279,521	261,497
Provision for loan losses	4,062	9,565	17,179	28,500
Noninterest income	20,831	21,094	60,606	62,014
Noninterest expense	64,083	60,620	189,096	185,825
Income before taxes	47,319	40,295	133,852	109,186
Income tax expense	5,381	8,454	19,032	23,297
Income from continuing operations	41,938	31,841	114,820	85,889
Average assets	9,838,000	9,710,000	9,788,000	9,723,000
Personal Banking [Member]
Segment Reporting Information [Line Items]
Net interest income	31,145	30,675	93,220	91,484
Provision for loan losses	1,333	1,622	4,544	5,399
Noninterest income	28,266	28,261	87,700	87,932
Noninterest expense	56,648	56,127	165,945	168,760
Income before taxes	1,430	1,187	10,431	5,257
Income tax expense	160	244	1,484	1,273
Income from continuing operations	1,270	943	8,947	3,984
Average assets	4,791,000	5,072,000	4,853,000	5,233,000
Institutional Banking [Member]
Segment Reporting Information [Line Items]
Net interest income	15,293	12,763	47,351	36,731
Provision for loan losses	355	313	1,027	1,101
Noninterest income	43,169	46,405	131,999	140,232
Noninterest expense	47,081	44,727	141,499	136,086
Income before taxes	11,026	14,128	36,824	39,776
Income tax expense	1,241	2,965	5,227	8,437
Income from continuing operations	9,785	11,163	31,597	31,339
Average assets	3,920,000	3,582,000	3,895,000	3,457,000
Healthcare Services [Member]
Segment Reporting Information [Line Items]
Net interest income	9,419	8,034	28,546	22,855
Noninterest income	8,619	8,546	26,394	27,351
Noninterest expense	12,573	10,347	36,939	31,899
Income before taxes	5,465	6,233	18,001	18,307
Income tax expense	609	1,308	2,559	3,900
Income from continuing operations	4,856	4,925	15,442	14,407
Average assets	$ 2,201,000	$ 1,949,000	$ 2,168,000	$ 1,875,000