Segment Reporting (Operating Results and Total Assets Reportable Segments) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Assets	$ 32,708,617		$ 32,708,617		$ 30,389,344
Net interest income	224,407	$ 241,787	455,208	$ 483,338
Non-interest income	60,076	75,853	133,454	144,465
Non-interest expense	176,584	180,640	355,420	356,326
Pre-tax, pre-provision net revenue	107,899	137,000	233,242	271,477
Provision for credit losses	40,000	11,900	116,000	20,500
Income before income tax expense	67,899	125,100	117,242	250,977
Income tax expense	14,802	26,451	25,946	52,592
Net income	53,097	98,649	91,296	198,385
Operating Segments [Member] | Commercial Banking [Member]
Segment Reporting Information [Line Items]
Assets	12,527,397		12,527,397		11,541,803
Net interest income	104,862	100,216	204,178	198,558
Non-interest income	14,725	14,645	27,964	28,656
Non-interest expense	44,694	46,196	91,238	90,814
Pre-tax, pre-provision net revenue	74,893	68,665	140,904	136,400
Provision for credit losses	37,559	7,741	101,083	13,982
Income before income tax expense	37,334	60,924	39,821	122,418
Income tax expense	9,143	15,110	9,752	30,361
Net income	28,191	45,814	30,069	92,057
Operating Segments [Member] | HSA Bank [Member]
Segment Reporting Information [Line Items]
Assets	77,992		77,992		80,176
Net interest income	39,334	44,013	82,007	87,111
Non-interest income	23,103	24,979	49,486	50,556
Non-interest expense	34,020	34,253	71,098	67,775
Pre-tax, pre-provision net revenue	28,417	34,739	60,395	69,892
Provision for credit losses	0	0	0	0
Income before income tax expense	28,417	34,739	60,395	69,892
Income tax expense	7,587	9,206	16,125	18,522
Net income	20,830	25,533	44,270	51,370
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Assets	10,150,431		10,150,431		9,348,727
Net interest income	104,870	107,838	204,340	214,128
Non-interest income	23,405	27,675	51,025	53,057
Non-interest expense	93,686	96,166	192,653	191,241
Pre-tax, pre-provision net revenue	34,589	39,347	62,712	75,944
Provision for credit losses	2,444	4,159	15,005	6,518
Income before income tax expense	32,145	35,188	47,707	69,426
Income tax expense	6,365	7,459	9,446	14,717
Net income	25,780	27,729	38,261	54,709
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Assets	9,952,797		9,952,797		$ 9,418,638
Net interest income	(24,659)	(10,280)	(35,317)	(16,459)
Non-interest income	(1,157)	8,554	4,979	12,196
Non-interest expense	4,184	4,025	431	6,496
Pre-tax, pre-provision net revenue	(30,000)	(5,751)	(30,769)	(10,759)
Provision for credit losses	(3)	0	(88)	0
Income before income tax expense	(29,997)	(5,751)	(30,681)	(10,759)
Income tax expense	(8,293)	(5,324)	(9,377)	(11,008)
Net income	$ (21,704)	$ (427)	$ (21,304)	$ 249