Other Comprehensive Income (Loss) - Other Comprehensive Income (Loss) Components and Related Tax Effects (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Income Loss [Line Items]
Balances at beginning of period	$ (16,108)	$ (29,097)	$ (19,998)	$ (41,665)
Balances at beginning of period, net of tax	(16,108)	(29,097)	(19,998)	(41,665)
Other comprehensive income before reclassifications	(5,937)	3,216	(2,040)	15,786
Reclassification adjustment for gains realized in income		(20)	(7)	(22)
Net current period other comprehensive income	(5,937)	3,196	(2,047)	15,764
Balances at end of period, net of tax	(22,045)	(25,901)	(22,045)	(25,901)
Unrealized Gains (Losses) on Securities Available for Sale [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balances at beginning of period	(425)	(27,825)	(4,315)	(40,393)
Income tax		14,138		14,138
Balances at beginning of period, net of tax	(425)	(13,687)	(4,315)	(26,255)
Other comprehensive income before reclassifications	(5,937)	3,216	(2,040)	15,786
Reclassification adjustment for gains realized in income		(20)	(7)	(22)
Net current period other comprehensive income	(5,937)	3,196	(2,047)	15,764
Balances at end of period, net of tax	(6,362)	(10,491)	(6,362)	(10,491)
Disproportionate Tax Effect from Securities Available for Sale [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balances at beginning of period	(17,110)	(2,972)	(17,110)	(2,972)
Income tax		(14,138)		(14,138)
Balances at beginning of period, net of tax	(17,110)	(17,110)	(17,110)	(17,110)
Balances at end of period, net of tax	(17,110)	(17,110)	(17,110)	(17,110)
Unrealized Gains (Losses) from Postretirement Plan [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balances at beginning of period	916	1,829	916	1,829
Income tax		(640)		(640)
Balances at beginning of period, net of tax	916	1,189	916	1,189
Balances at end of period, net of tax	916	1,189	916	1,189
Disproportionate Tax Effect from Postretirement Plan [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balances at beginning of period	511	(129)	511	(129)
Income tax		640		640
Balances at beginning of period, net of tax	511	511	511	511
Balances at end of period, net of tax	$ 511	$ 511	$ 511	$ 511