Segment Reporting (Operating Results and Total Assets Reportable Segments) (Detail) - USD ($) $ in Thousands	3 Months Ended								9 Months Ended		12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total Assets	$ 30,389,344				$ 27,610,315						$ 30,389,344	$ 27,610,315
Net interest income (expense)	231,250	$ 240,539	$ 241,787	$ 241,551	237,131	$ 230,372	$ 225,010	$ 214,168			955,127	906,681	$ 796,287
Noninterest Income									$ 285,315	$ 282,568	285,315	282,568	259,478
Non-interest expense	179,730	179,894	180,640	175,686	174,759	178,783	180,459	171,615			715,950	705,616	661,075
Pre-tax, pre-provision net revenue											524,492	483,633	394,690
Provision for Loan and Lease Losses	6,000	11,300	11,900	8,600	10,000	10,500	10,500	11,000			37,800	42,000	40,900
Income (loss) before income tax expense	116,439	119,276	125,100	125,877	125,535	113,373	102,425	100,300			486,692	441,633	353,790
Income tax expense	25,966	25,411	26,451	26,141	26,697	13,700	20,743	20,075			103,969	81,215	98,351
Net income	90,473	$ 93,865	$ 98,649	$ 99,736	98,838	$ 99,673	$ 81,682	$ 80,225			382,723	360,418	255,439
Operating Segments [Member] | Commercial Banking [Member]
Segment Reporting Information [Line Items]
Total Assets	11,541,803				10,477,050						11,541,803	10,477,050
Net interest income (expense)											372,845	356,509	322,393
Noninterest Income									59,063	64,765	59,063	64,765	55,194
Non-interest expense											181,580	174,054	154,037
Pre-tax, pre-provision net revenue											250,328	247,220	223,550
Provision for Loan and Lease Losses											25,407	32,388	34,066
Income (loss) before income tax expense											224,921	214,832	189,484
Income tax expense											55,331	52,849	52,676
Net income											169,590	161,983	136,808
Operating Segments [Member] | HSA Bank [Member]
Segment Reporting Information [Line Items]
Total Assets	80,176				70,826						80,176	70,826
Net interest income (expense)											167,239	143,255	104,704
Noninterest Income									97,041	89,323	97,041	89,323	77,378
Non-interest expense											135,586	124,594	113,143
Pre-tax, pre-provision net revenue											128,694	107,984	68,939
Provision for Loan and Lease Losses											0	0	0
Income (loss) before income tax expense											128,694	107,984	68,939
Income tax expense											33,460	28,076	19,165
Net income											95,234	79,908	49,774
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Total Assets	9,348,727				8,727,335						9,348,727	8,727,335
Net interest income (expense)											400,744	404,869	383,700
Noninterest Income									109,270	109,669	109,270	109,669	107,368
Non-interest expense											388,399	384,599	373,081
Pre-tax, pre-provision net revenue											121,615	129,939	117,987
Provision for Loan and Lease Losses											12,393	9,612	6,834
Income (loss) before income tax expense											109,222	120,327	111,153
Income tax expense											21,735	23,945	30,899
Net income											87,487	96,382	80,254
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total Assets	$ 9,418,638				$ 8,335,104						9,418,638	8,335,104
Net interest income (expense)											14,299	2,048	(14,510)
Noninterest Income									$ 19,941	$ 18,811	19,941	18,811	19,538
Non-interest expense											10,385	22,369	20,814
Pre-tax, pre-provision net revenue											23,855	(1,510)	(15,786)
Provision for Loan and Lease Losses											0	0	0
Income (loss) before income tax expense											23,855	(1,510)	(15,786)
Income tax expense											(6,557)	(23,655)	(4,389)
Net income											$ 30,412	$ 22,145	$ (11,397)