Segment Reporting - Operating Results and Total Assets Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net interest income (loss)	$ 204,932	$ 200,904	$ 197,787	$ 192,664	$ 185,259	$ 180,197	$ 176,905	$ 176,152	$ 796,287	$ 718,513	$ 664,625
Provision for loan and lease losses	13,000	10,150	7,250	10,500	12,500	14,250	14,000	15,600	40,900	56,350	49,300
Net interest income (loss) after provision for loan and lease losses									755,387	662,163	615,325
Non-interest income	66,039								259,478	264,478	237,777
Non-interest expense	171,049	161,823	164,419	163,784	161,871	156,097	152,778	152,445	661,075	623,191	555,341
Income before income tax expense	86,922	94,777	90,669	81,422	81,505	76,262	75,202	70,481	353,790	303,450	297,761
Income tax expense (benefit)	17,029	30,281	29,090	21,951	23,845	24,445	24,599	23,434	98,351	96,323	93,032
Net income	69,893	$ 64,496	$ 61,579	$ 59,471	57,660	$ 51,817	$ 50,603	$ 47,047	255,439	207,127	204,729
Total Assets	26,487,645				26,072,529				26,487,645	26,072,529
Operating Segments [Member] | Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									322,393	287,596	266,085
Provision for loan and lease losses									38,518	37,455	30,546
Net interest income (loss) after provision for loan and lease losses									283,875	250,141	235,539
Non-interest income									55,194	57,253	46,967
Non-interest expense									154,037	138,379	129,499
Income before income tax expense									185,032	169,015	153,007
Income tax expense (benefit)									51,438	53,649	47,804
Net income									133,594	115,366	105,203
Total Assets	9,350,028				9,069,445				9,350,028	9,069,445
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									383,700	367,137	356,881
Provision for loan and lease losses									2,382	18,895	18,754
Net interest income (loss) after provision for loan and lease losses									381,318	348,242	338,127
Non-interest income									107,368	110,197	108,647
Non-interest expense									373,081	369,132	335,834
Income before income tax expense									115,605	89,307	110,940
Income tax expense (benefit)									32,137	28,348	34,605
Net income									83,468	60,959	76,335
Total Assets	8,909,671				8,721,046				8,909,671	8,721,046
Operating Segments [Member] | HSA Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									104,704	81,451	73,433
Provision for loan and lease losses									0	0	0
Net interest income (loss) after provision for loan and lease losses									104,704	81,451	73,433
Non-interest income									77,378	71,710	62,475
Non-interest expense									113,143	97,152	81,449
Income before income tax expense									68,939	56,009	54,459
Income tax expense (benefit)									19,165	17,779	17,016
Net income									49,774	38,230	37,443
Total Assets	76,308				83,987				76,308	83,987
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									(14,510)	(17,671)	(31,774)
Provision for loan and lease losses									0	0	0
Net interest income (loss) after provision for loan and lease losses									(14,510)	(17,671)	(31,774)
Non-interest income									19,538	25,318	19,688
Non-interest expense									20,814	18,528	8,559
Income before income tax expense									(15,786)	(10,881)	(20,645)
Income tax expense (benefit)									(4,389)	(3,453)	(6,393)
Net income									(11,397)	(7,428)	$ (14,252)
Total Assets	$ 8,151,638				$ 8,198,051				$ 8,151,638	$ 8,198,051