Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net periodic pension cost									$ (5,781)	$ 7,363	$ (1,529)
Interest income/(expense)	90,043	108,367	84,482	83,478	81,187	77,996	79,191	79,050	366,370	317,424	308,757
Impairment losses									(100)		(165)
Amortization of net actuarial losses									1,933	3,436	5,197
Tax (expense) or benefit	(10,302)	(11,095)	(7,658)	(9,242)	(10,602)	(6,869)	(13,734)	(10,392)	(38,297)	(41,597)	(36,110)
Net income	29,250	29,134	18,773	26,510	24,549	23,948	28,478	23,945	103,667	100,920	91,675
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income									7,581	(4,105)	7,057
Reclassification 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									9,830	4,341	15,052
Impairment losses									(100)	(1,000)	(1,414)
Total before tax									9,730	3,341	13,638
Tax (expense) or benefit									(3,699)	(1,137)	(5,272)
Net income									6,031	2,204	8,366
Reclassification 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)									(1,437)	(225)	870
Tax (expense) or benefit									446	37	(348)
Net income									(991)	(188)	522
Reclassification 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									94		(95)
Net income									(154)		145
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member] | Accumulated Gains and Losses on Cash Flow Hedges [Member] | Interest Rate Contract [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income/(expense)									(248)		240
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member] | Accumulated Defined Benefit Pensions Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net periodic pension cost									6,266	(6,901)	1,903
Amortization of net actuarial losses									(1,933)	(3,436)	(5,197)
Tax (expense) or benefit									(1,638)	4,216	1,318
Net income									$ 2,695	$ (6,121)	$ (1,976)