Segment Reporting (Operating Results and Total Assets Reportable Segments) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total Assets	$ 28,942,043		$ 28,942,043		$ 27,610,315
Net interest income (expense)	241,787	$ 225,010	483,338	$ 439,178
Provision for Loan and Lease Losses	11,900	10,500	20,500	21,500
Net interest income after provision for loan and lease losses	229,887	214,510	462,838	417,678
Noninterest Income	75,853	68,374	144,465	137,121
Total non-interest expense	180,640	180,459	356,326	352,074
Income (loss) before income tax expense	125,100	102,425	250,977	202,725
Income tax expense	26,451	20,743	52,592	40,818
Net income	98,649	81,682	198,385	161,907
Operating Segments [Member] | Commercial Banking [Member]
Segment Reporting Information [Line Items]
Total Assets	11,061,635		11,061,635		10,477,050
Net interest income (expense)	92,171	88,459	182,681	173,110
Provision for Loan and Lease Losses	7,741	8,630	13,982	15,477
Net interest income after provision for loan and lease losses	84,430	79,829	168,699	157,633
Noninterest Income	14,645	15,041	28,656	30,357
Total non-interest expense	46,196	42,979	90,814	84,224
Income (loss) before income tax expense	52,879	51,891	106,541	103,766
Income tax expense	13,008	12,765	26,209	25,527
Net income	39,871	39,126	80,332	78,239
Operating Segments [Member] | HSA Bank [Member]
Segment Reporting Information [Line Items]
Total Assets	82,232		82,232		70,826
Net interest income (expense)	42,626	35,265	84,367	68,189
Provision for Loan and Lease Losses	0	0	0	0
Net interest income after provision for loan and lease losses	42,626	35,265	84,367	68,189
Noninterest Income	24,979	22,882	50,556	45,551
Total non-interest expense	34,253	31,220	67,775	62,735
Income (loss) before income tax expense	33,352	26,927	67,148	51,005
Income tax expense	8,672	7,001	17,459	13,261
Net income	24,680	19,926	49,689	37,744
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Total Assets	9,059,657		9,059,657		8,727,335
Net interest income (expense)	102,699	101,902	204,059	200,830
Provision for Loan and Lease Losses	4,159	1,870	6,518	6,023
Net interest income after provision for loan and lease losses	98,540	100,032	197,541	194,807
Noninterest Income	27,675	26,378	53,057	51,573
Total non-interest expense	96,166	95,197	191,241	192,026
Income (loss) before income tax expense	30,049	31,213	59,357	54,354
Income tax expense	5,980	6,211	11,812	10,816
Net income	24,069	25,002	47,545	43,538
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total Assets	8,738,519		8,738,519		$ 8,335,104
Net interest income (expense)	4,291	(616)	12,231	(2,951)
Provision for Loan and Lease Losses	0	0	0	0
Net interest income after provision for loan and lease losses	4,291	(616)	12,231	(2,951)
Noninterest Income	8,554	4,073	12,196	9,640
Total non-interest expense	4,025	11,063	6,496	13,089
Income (loss) before income tax expense	8,820	(7,606)	17,931	(6,400)
Income tax expense	(1,209)	(5,234)	(2,888)	(8,786)
Net income	$ 10,029	$ (2,372)	$ 20,819	$ 2,386