Accumulated Other Comprehensive Income (Loss) - Schedule of Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI, Beginning Balance	$ 20,818	$ 18,727	$ 29,801	$ 15,004
Other comprehensive income (loss) before reclassifications	(44,818)	6,952	(53,564)	10,664
Amounts reclassified from accumulated other comprehensive income (loss)	(1,021)	(3,337)	(1,258)	(3,326)
Other comprehensive income (loss), net of tax	(45,839)	3,615	(54,822)	7,338
AOCI, Ending Balance	(25,021)	22,342	(25,021)	22,342
Accumulated Net Unrealized Gains and Losses on Available - for - Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI, Beginning Balance	29,661	28,013	39,054	24,612
Other comprehensive income (loss) before reclassifications	(45,343)	6,952	(54,089)	10,664
Amounts reclassified from accumulated other comprehensive income (loss)	(1,091)	(3,802)	(1,738)	(4,113)
Other comprehensive income (loss), net of tax	(46,434)	3,150	(55,827)	6,551
AOCI, Ending Balance	(16,773)	31,163	(16,773)	31,163
Accumulated Net Unrealized Gains and Losses on Held-to-Maturity Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI, Beginning Balance	3,163	4,607	3,269	4,745
Other comprehensive income (loss) before reclassifications
Amounts reclassified from accumulated other comprehensive income (loss)	(107)	(139)	(213)	(277)
Other comprehensive income (loss), net of tax	(107)	(139)	(213)	(277)
AOCI, Ending Balance	3,056	4,468	3,056	4,468
Accumulated Net Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI, Beginning Balance		1		145
Other comprehensive income (loss) before reclassifications	525		525
Amounts reclassified from accumulated other comprehensive income (loss)				(144)
Other comprehensive income (loss), net of tax	525		525	(144)
AOCI, Ending Balance	525	1	525	1
Accumulated Defined Benefit Pensions Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI, Beginning Balance	(12,006)	(13,894)	(12,522)	(14,498)
Other comprehensive income (loss) before reclassifications
Amounts reclassified from accumulated other comprehensive income (loss)	177	604	693	1,208
Other comprehensive income (loss), net of tax	177	604	693	1,208
AOCI, Ending Balance	$ (11,829)	$ (13,290)	$ (11,829)	$ (13,290)