Segment Reporting (Operating Results and Total Assets Reportable Segments) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Assets	$ 32,994,443		$ 32,994,443		$ 30,389,344
Net interest income	219,256	$ 240,539	674,464	$ 723,877
Non-interest income	75,060	69,931	208,514	214,396
Non-interest expense	183,996	179,894	539,416	536,220
Pre-tax, pre-provision net revenue	110,320	130,576	343,562	402,053
Provision for credit losses	22,750	11,300	138,750	31,800
Income before income tax expense	87,570	119,276	204,812	370,253
Income tax expense	18,289	25,411	44,235	78,003
Net income	69,281	93,865	160,577	292,250
Operating Segments [Member] | Commercial Banking [Member]
Segment Reporting Information [Line Items]
Assets	12,666,764		12,666,764		11,541,803
Net interest income	107,417	104,549	311,595	303,107
Non-interest income	13,099	13,987	41,063	42,643
Non-interest expense	47,610	45,261	138,848	136,075
Pre-tax, pre-provision net revenue	72,906	73,275	213,810	209,675
Provision for credit losses	30,793	9,312	131,876	23,294
Income before income tax expense	42,113	63,963	81,934	186,381
Income tax expense	10,314	15,863	20,066	46,224
Net income	31,799	48,100	61,868	140,157
Operating Segments [Member] | HSA Bank [Member]
Segment Reporting Information [Line Items]
Assets	83,104		83,104		80,176
Net interest income	39,861	43,581	121,868	130,692
Non-interest income	27,235	23,526	76,721	74,082
Non-interest expense	34,789	32,918	105,887	100,693
Pre-tax, pre-provision net revenue	32,307	34,189	92,702	104,081
Provision for credit losses	0	0	0	0
Income before income tax expense	32,307	34,189	92,702	104,081
Income tax expense	8,626	9,060	24,751	27,582
Net income	23,681	25,129	67,951	76,499
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Assets	10,039,505		10,039,505		9,348,727
Net interest income	108,218	104,613	312,558	318,741
Non-interest income	28,970	28,115	79,995	81,172
Non-interest expense	98,991	99,835	291,644	291,076
Pre-tax, pre-provision net revenue	38,197	32,893	100,909	108,837
Provision for credit losses	(8,040)	1,988	6,965	8,506
Income before income tax expense	46,237	30,905	93,944	100,331
Income tax expense	9,155	6,552	18,601	21,269
Net income	37,082	24,353	75,343	79,062
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Assets	10,205,070		10,205,070		$ 9,418,638
Net interest income	(36,240)	(12,204)	(71,557)	(28,663)
Non-interest income	5,756	4,303	10,735	16,499
Non-interest expense	2,606	1,880	3,037	8,376
Pre-tax, pre-provision net revenue	(33,090)	(9,781)	(63,859)	(20,540)
Provision for credit losses	(3)	0	(91)	0
Income before income tax expense	(33,087)	(9,781)	(63,768)	(20,540)
Income tax expense	(9,806)	(6,064)	(19,183)	(17,072)
Net income	$ (23,281)	$ (3,717)	$ (44,585)	$ (3,468)