Schedule of Business Segment Financial Results (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Net interest income	$ 150,226	$ 137,394	$ 298,148	$ 271,709
Provision for loan losses	7,000	14,500	17,000	23,500
Noninterest income	100,289	110,306	205,814	213,223
Noninterest expense	177,218	176,939	353,094	350,749
Income before taxes	66,297	56,261	133,868	110,683
Income tax expense	10,873	11,490	20,911	23,936
Income from continuing operations	55,424	44,771	112,957	86,747
Average assets	20,620,000	20,315,000	20,683,000	20,275,000
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net interest income	93,011	86,810	185,006	172,108
Provision for loan losses	5,139	12,114	13,117	18,934
Noninterest income	19,178	21,714	39,775	40,919
Noninterest expense	62,859	62,545	125,131	125,202
Income before taxes	44,191	33,865	86,533	68,891
Income tax expense	7,246	7,188	13,516	14,870
Income from continuing operations	36,945	26,677	73,017	54,021
Average assets	9,740,000	9,842,000	9,765,000	9,730,000
Personal Banking [Member]
Segment Reporting Information [Line Items]
Net interest income	31,111	30,615	62,003	60,809
Provision for loan losses	1,539	2,066	3,211	3,777
Noninterest income	28,720	31,135	59,434	59,671
Noninterest expense	54,259	56,516	109,225	112,632
Income before taxes	4,033	3,168	9,001	4,071
Income tax expense	662	106	1,406	1,015
Income from continuing operations	3,371	3,062	7,595	3,056
Average assets	4,799,000	5,293,000	4,885,000	5,317,000
Institutional Banking [Member]
Segment Reporting Information [Line Items]
Net interest income	16,279	12,415	32,023	23,971
Provision for loan losses	322	320	672	789
Noninterest income	43,411	48,161	88,830	93,828
Noninterest expense	47,551	46,610	94,383	91,362
Income before taxes	11,817	13,646	25,798	25,648
Income tax expense	1,939	3,137	4,030	5,456
Income from continuing operations	9,878	10,509	21,768	20,192
Average assets	3,934,000	3,316,000	3,882,000	3,391,000
Healthcare Services [Member]
Segment Reporting Information [Line Items]
Net interest income	9,825	7,554	19,116	14,821
Noninterest income	8,980	9,296	17,775	18,805
Noninterest expense	12,549	11,268	24,355	21,553
Income before taxes	6,256	5,582	12,536	12,073
Income tax expense	1,026	1,059	1,959	2,595
Income from continuing operations	5,230	4,523	10,577	9,478
Average assets	$ 2,147,000	$ 1,864,000	$ 2,151,000	$ 1,837,000