Accumulated Other Comprehensive Income (Loss) - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income/(expense)	$ 90,993	$ 83,478
Impairment losses		(100)
Tax (expense) or benefit	(9,225)	(9,242)
Net income	20,906	26,510
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	713	(321)
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Accumulated Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	2,683	559
Impairment losses		(100)
Total before tax	2,683	459
Tax (expense) or benefit	(995)	(177)
Net income	1,688	282
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Accumulated Unrealized Gains Losses on Held-to-Maturity Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income/(expense)	(337)	(397)
Tax (expense) or benefit	(66)	127
Net income	(403)	(270)
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Accumulated Gains and Losses on Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax (expense) or benefit	71
Net income	(115)
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Accumulated Gains and Losses on Cash Flow Hedges [Member] | Interest Rate Contracts [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income/(expense)	(186)
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Accumulated Defined Benefit Pensions Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits	(738)	(352)
Tax (expense) or benefit	281	19
Net income	$ (457)	$ (333)