Schedule of Segment Financial Results (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Net interest income	$ 140,858	$ 124,765	$ 412,567	$ 363,867
Provision for loan losses	11,500	13,000	35,000	25,000
Noninterest income	104,306	103,542	317,529	304,281
Noninterest expense	171,821	165,211	522,570	498,898
Income before taxes	61,843	50,096	172,526	144,250
Income tax expense	12,971	10,674	36,907	34,016
Income from continuing operations	48,872	39,422	135,619	110,234
Average assets	20,313,000	19,692,000	20,288,000	19,488,000
Bank [Member]
Segment Reporting Information [Line Items]
Net interest income	137,331	121,963	403,450	355,847
Provision for loan losses	11,500	13,000	35,000	25,000
Noninterest income	79,759	80,540	245,583	234,643
Noninterest expense	150,057	144,450	456,680	435,708
Income before taxes	55,533	45,053	157,353	129,782
Income tax expense	11,649	9,560	33,733	30,478
Income from continuing operations	43,884	35,493	123,620	99,304
Average assets	19,542,050	18,441,950	19,501,000	18,205,900
Asset Servicing [Member]
Segment Reporting Information [Line Items]
Net interest income	3,527	2,802	9,117	8,020
Noninterest income	24,547	23,002	71,946	69,638
Noninterest expense	21,764	20,761	65,890	63,190
Income before taxes	6,310	5,043	15,173	14,468
Income tax expense	1,322	1,114	3,174	3,538
Income from continuing operations	4,988	3,929	11,999	10,930
Average assets	$ 770,950	$ 1,250,050	$ 787,000	$ 1,282,100