Accumulated Other Comprehensive Income (Loss) - Components of Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), Beginning balance	$ (22,925)	$ (26,510)
Other comprehensive income before income taxes:
Net change in unrealized gain (loss)	(12,438)	8,018
Reclassification adjustment for net (gains) losses realized and included in earnings	1,929	1,829
Amortization of unrealized net gain on securities transferred to held-to-maturity	(2,984)
Income tax expense (benefit)	(4,940)	3,522
Accumulated other comprehensive income (loss), Ending balance	(31,478)	(20,185)
Available for Sale Securities [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), Beginning balance	38,854	60,478
Other comprehensive income before income taxes:
Net change in unrealized gain (loss)	(12,434)	8,409
Transfer of net unrealized gain from AFS to HTM, net of cumulative tax effect		(24,598)
Reclassification adjustment for net (gains) losses realized and included in earnings		(12)
Income tax expense (benefit)	(4,586)	2,983
Accumulated other comprehensive income (loss), Ending balance	31,006	41,294
Held to Maturity Securities Transferred from AFS [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), Beginning balance	19,090
Other comprehensive income before income taxes:
Transfer of net unrealized gain from AFS to HTM, net of cumulative tax effect		24,598
Amortization of unrealized net gain on securities transferred to held-to-maturity	(2,984)
Income tax expense (benefit)	(1,078)
Accumulated other comprehensive income (loss), Ending balance	17,184	24,598
Employee Benefit Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), Beginning balance	(80,688)	(86,923)
Other comprehensive income before income taxes:
Reclassification adjustment for net (gains) losses realized and included in earnings	1,754	1,753
Income tax expense (benefit)	657	657
Accumulated other comprehensive income (loss), Ending balance	(79,591)	(85,827)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), Beginning balance	(181)	(65)
Other comprehensive income before income taxes:
Net change in unrealized gain (loss)	(4)	(391)
Reclassification adjustment for net (gains) losses realized and included in earnings	175	88
Income tax expense (benefit)	67	(118)
Accumulated other comprehensive income (loss), Ending balance	$ (77)	$ (250)