Segment Reporting (Operating Results and Total Assets Reportable Segments) (Detail) - USD ($) $ in Thousands	3 Months Ended								9 Months Ended		12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total Assets	$ 32,590,690				$ 30,389,344						$ 32,590,690	$ 30,389,344
Net interest income (expense)	216,929	$ 219,256	$ 224,407	$ 230,801	231,250	$ 240,539	$ 241,787	$ 241,551			891,393	955,127	$ 906,681
Noninterest Income									$ 285,277	$ 285,315	285,277	285,315	282,568
Non-interest expense	219,530	183,996	176,584	178,836	179,730	179,894	180,640	175,686			758,946	715,950	705,616
Pre-tax, pre-provision net revenue											417,724	524,492	483,633
Provision for credit losses	(1,000)	22,750	40,000	76,000	6,000	11,300	11,900	8,600			137,750	37,800	42,000
Provision for Loan and Lease Losses											137,848	37,800	42,000
Income (loss) before income tax expense	75,162	87,570	67,899	49,343	116,439	119,276	125,100	125,877			279,974	486,692	441,633
Income tax expense	15,118	18,289	14,802	11,144	25,966	25,411	26,451	26,141			59,353	103,969	81,215
Net income	60,044	$ 69,281	$ 53,097	$ 38,199	90,473	$ 93,865	$ 98,649	$ 99,736			220,621	382,723	360,418
Operating Segments [Member] | Commercial Banking [Member]
Segment Reporting Information [Line Items]
Total Assets	12,689,255				11,541,803						12,689,255	11,541,803
Net interest income (expense)											423,869	403,258	381,098
Noninterest Income									58,366	59,063	58,366	59,063	64,765
Non-interest expense											187,572	181,580	174,054
Pre-tax, pre-provision net revenue											294,663	280,741	271,809
Provision for Loan and Lease Losses											137,993	25,407	32,388
Income (loss) before income tax expense											156,670	255,334	239,421
Income tax expense											38,368	63,323	58,898
Net income											118,302	192,011	180,523
Operating Segments [Member] | HSA Bank [Member]
Segment Reporting Information [Line Items]
Total Assets	80,352				80,176						80,352	80,176
Net interest income (expense)											162,363	172,685	147,920
Noninterest Income									100,826	97,041	100,826	97,041	89,323
Non-interest expense											140,637	135,586	124,594
Pre-tax, pre-provision net revenue											122,552	134,140	112,649
Provision for Loan and Lease Losses											0	0	0
Income (loss) before income tax expense											122,552	134,140	112,649
Income tax expense											32,721	35,547	29,289
Net income											89,831	98,593	83,360
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Total Assets	9,769,824				9,348,727						9,769,824	9,348,727
Net interest income (expense)											422,979	420,898	422,063
Noninterest Income									106,279	109,270	106,279	109,270	109,669
Non-interest expense											390,596	388,399	384,603
Pre-tax, pre-provision net revenue											138,662	141,769	147,129
Provision for Loan and Lease Losses											(144)	12,393	9,612
Income (loss) before income tax expense											138,806	129,376	137,517
Income tax expense											27,484	27,428	27,366
Net income											111,322	101,948	110,151
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total Assets	$ 10,051,259				$ 9,418,638						10,051,259	9,418,638
Net interest income (expense)											(117,818)	(41,714)	(44,400)
Noninterest Income									$ 19,806	$ 19,941	19,806	19,941	18,811
Non-interest expense											40,141	10,385	22,365
Pre-tax, pre-provision net revenue											(138,153)	(32,158)	(47,954)
Provision for Loan and Lease Losses											(99)	0	0
Income (loss) before income tax expense											(138,054)	(32,158)	(47,954)
Income tax expense											(39,220)	(22,329)	(34,338)
Net income											$ (98,834)	$ (9,829)	$ (13,616)