Segment Reporting (Operating Results and Total Assets Reportable Segments) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Goodwill	$ 538,373		$ 538,373		$ 538,373
Assets	33,753,752		33,753,752		32,590,690
Net interest income	220,852	$ 224,407	444,616	$ 455,208
Non-interest income	72,702	60,076	149,459	133,454
Non-interest expense	187,028	176,584	375,010	355,420
Pre-tax, pre-provision net revenue	106,526	107,899	219,065	233,242
Provision for credit losses	(21,500)	40,000	(47,250)	116,000
Income before income tax expense	128,026	67,899	266,315	117,242
Income tax expense	33,991	14,802	64,202	25,946
Net income	94,035	53,097	202,113	91,296
Operating Segments [Member] | Commercial Banking
Segment Reporting Information [Line Items]
Goodwill	131,000		131,000		131,000
Assets	14,837,710		14,837,710		14,732,792
Net interest income	141,124	128,123	283,162	245,710
Non-interest income	25,713	21,849	50,890	44,265
Non-interest expense	61,445	61,261	126,281	126,482
Pre-tax, pre-provision net revenue	105,392	88,711	207,771	163,493
Provision for credit losses	(23,328)	43,569	(42,701)	112,587
Income before income tax expense	128,720	45,142	250,472	50,906
Income tax expense	32,566	11,015	63,369	12,421
Net income	96,154	34,127	187,103	38,485
Operating Segments [Member] | HSA Bank
Segment Reporting Information [Line Items]
Goodwill	21,813		21,813		21,813
Assets	78,054		78,054		80,352
Net interest income	42,193	39,334	84,302	82,007
Non-interest income	26,554	23,103	53,559	49,486
Non-interest expense	32,792	34,020	69,042	71,098
Pre-tax, pre-provision net revenue	35,955	28,417	68,819	60,395
Provision for credit losses	0	0	0	0
Income before income tax expense	35,955	28,417	68,819	60,395
Income tax expense	9,600	7,587	18,375	16,125
Net income	26,355	20,830	50,444	44,270
Operating Segments [Member] | Retail Banking
Segment Reporting Information [Line Items]
Goodwill	385,560		385,560		385,560
Assets	7,432,340		7,432,340		7,726,287
Net interest income	92,540	81,609	181,353	162,808
Non-interest income	16,763	16,281	32,834	34,724
Non-interest expense	72,346	77,119	148,470	157,409
Pre-tax, pre-provision net revenue	36,957	20,771	65,717	40,123
Provision for credit losses	1,754	(3,566)	(4,632)	3,501
Income before income tax expense	35,203	24,337	70,349	36,622
Income tax expense	7,745	5,305	15,477	7,983
Net income	27,458	19,032	54,872	28,639
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Goodwill	0		0		0
Assets	11,405,648		11,405,648		$ 10,051,259
Net interest income	(55,005)	(24,659)	(104,201)	(35,317)
Non-interest income	3,672	(1,157)	12,176	4,979
Non-interest expense	20,445	4,184	31,217	431
Pre-tax, pre-provision net revenue	(71,778)	(30,000)	(123,242)	(30,769)
Provision for credit losses	74	(3)	83	(88)
Income before income tax expense	(71,852)	(29,997)	(123,325)	(30,681)
Income tax expense	(15,920)	(9,105)	(33,019)	(10,583)
Net income	$ (55,932)	$ (20,892)	$ (90,306)	$ (20,098)