Segment Reporting (Operating Results and Total Assets Reportable Segments) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total Assets	$ 26,174,930		$ 26,174,930		$ 26,072,529
Net interest income (expense)	197,787	$ 176,905	390,451	$ 353,057
Provision for Loan and Lease Losses	7,250	14,000	17,750	29,600
Interest Income (Expense), after Provision for Loan Loss	190,537	162,905	372,701	323,457
Noninterest Income	64,551	65,075	127,593	127,449
Noninterest Expense	164,419	152,778	328,203	305,223
Income (loss) before income tax expense	90,669	75,202	172,091	145,683
Income tax expense	29,090	24,599	51,041	48,033
Net income	61,579	50,603	121,050	97,650
Operating Segments [Member] | Commercial Banking [Member]
Segment Reporting Information [Line Items]
Total Assets	9,433,770		9,433,770		9,069,445
Net interest income (expense)	78,946	68,862	157,193	137,157
Provision for Loan and Lease Losses	10,692	11,612	17,489	21,889
Interest Income (Expense), after Provision for Loan Loss	68,254	57,250	139,704	115,268
Noninterest Income	12,532	14,755	25,956	25,903
Noninterest Expense	37,304	33,483	75,428	67,543
Income (loss) before income tax expense	43,482	38,522	90,232	73,628
Income tax expense	14,158	12,603	26,762	24,276
Net income	29,324	25,919	63,470	49,352
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Total Assets	8,802,060		8,802,060		8,721,046
Net interest income (expense)	95,902	91,620	189,492	182,191
Provision for Loan and Lease Losses	(3,442)	2,388	261	7,711
Interest Income (Expense), after Provision for Loan Loss	99,344	89,232	189,231	174,480
Noninterest Income	28,058	27,468	53,437	54,118
Noninterest Expense	94,322	91,526	189,501	183,537
Income (loss) before income tax expense	33,080	25,174	53,167	45,061
Income tax expense	10,353	8,617	15,769	14,857
Net income	22,727	16,557	37,398	30,204
Operating Segments [Member] | HSA Bank [Member]
Segment Reporting Information [Line Items]
Total Assets	78,569		78,569		83,987
Net interest income (expense)	25,574	20,005	49,626	39,924
Provision for Loan and Lease Losses	0	0	0	0
Interest Income (Expense), after Provision for Loan Loss	25,574	20,005	49,626	39,924
Noninterest Income	19,750	18,114	39,021	38,069
Noninterest Expense	28,750	24,688	56,989	48,945
Income (loss) before income tax expense	16,574	13,431	31,658	29,048
Income tax expense	5,323	4,384	9,389	9,577
Net income	11,251	9,047	22,269	19,471
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total Assets	7,860,531		7,860,531		$ 8,198,051
Net interest income (expense)	(2,635)	(3,582)	(5,860)	(6,215)
Provision for Loan and Lease Losses	0	0	0	0
Interest Income (Expense), after Provision for Loan Loss	(2,635)	(3,582)	(5,860)	(6,215)
Noninterest Income	4,211	4,738	9,179	9,359
Noninterest Expense	4,043	3,081	6,285	5,198
Income (loss) before income tax expense	(2,467)	(1,925)	(2,966)	(2,054)
Income tax expense	(744)	(1,005)	(879)	(677)
Net income	$ (1,723)	$ (920)	$ (2,087)	$ (1,377)