Segment Reporting (Operating Results and Total Assets Reportable Segments) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total Assets	$ 29,895,100		$ 29,895,100		$ 27,610,315
Net interest income (expense)	240,539	$ 230,372	723,877	$ 669,550
Noninterest Income	69,931	72,284	214,396	209,405
Total non-interest expense	179,894	178,783	536,220	530,857
Pre-tax, pre-provision net revenue	130,576	123,873	402,053	348,098
Provision for Loan and Lease Losses	11,300	10,500	31,800	32,000
Income (loss) before income tax expense	119,276	113,373	370,253	316,098
Income tax expense	25,411	13,700	78,003	54,518
Net income	93,865	99,673	292,250	261,580
Operating Segments [Member] | Commercial Banking [Member]
Segment Reporting Information [Line Items]
Total Assets	11,174,036		11,174,036		10,477,050
Net interest income (expense)	96,817	91,243	279,498	264,353
Noninterest Income	13,987	18,305	42,643	48,662
Total non-interest expense	45,261	44,506	136,075	128,730
Pre-tax, pre-provision net revenue	65,543	65,042	186,066	184,285
Provision for Loan and Lease Losses	9,312	9,991	23,294	25,468
Income (loss) before income tax expense	56,231	55,051	162,772	158,817
Income tax expense	13,828	13,542	40,037	39,069
Net income	42,403	41,509	122,735	119,748
Operating Segments [Member] | HSA Bank [Member]
Segment Reporting Information [Line Items]
Total Assets	75,636		75,636		70,826
Net interest income (expense)	42,206	36,731	126,573	104,920
Noninterest Income	23,526	22,159	74,082	67,710
Total non-interest expense	32,918	30,753	100,693	93,488
Pre-tax, pre-provision net revenue	32,814	28,137	99,962	79,142
Provision for Loan and Lease Losses	0	0	0	0
Income (loss) before income tax expense	32,814	28,137	99,962	79,142
Income tax expense	8,531	7,316	25,990	20,577
Net income	24,283	20,821	73,972	58,565
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Total Assets	9,228,537		9,228,537		8,727,335
Net interest income (expense)	99,459	101,952	303,518	302,782
Noninterest Income	28,115	26,848	81,172	78,421
Total non-interest expense	99,835	95,769	291,076	287,795
Pre-tax, pre-provision net revenue	27,739	33,031	93,614	93,408
Provision for Loan and Lease Losses	1,988	509	8,506	6,532
Income (loss) before income tax expense	25,751	32,522	85,108	86,876
Income tax expense	5,128	6,472	16,940	17,288
Net income	20,623	26,050	68,168	69,588
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total Assets	9,416,891		9,416,891		$ 8,335,104
Net interest income (expense)	2,057	446	14,288	(2,505)
Noninterest Income	4,303	4,972	16,499	14,612
Total non-interest expense	1,880	7,755	8,376	20,844
Pre-tax, pre-provision net revenue	4,480	(2,337)	22,411	(8,737)
Provision for Loan and Lease Losses	0	0	0	0
Income (loss) before income tax expense	4,480	(2,337)	22,411	(8,737)
Income tax expense	(2,076)	(13,630)	(4,964)	(22,416)
Net income	$ 6,556	$ 11,293	$ 27,375	$ 13,679