Supplemental Comprehensive Income Disclosures - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Comprehensive Income [Line Items]
Unrealized holding (losses) gains on securities arising during the period, Pretax Amount	$ 25,153	$ (34,975)	$ (661)
Less: reclassification adjustment for (gains) losses on securities included in net income, Pretax Amount	(550)	1,158	(192)
Total Unrealized (Losses) Gains on Securities, before Tax	24,603	(33,817)	(853)
Transition obligation	0	0	2
Net (loss) gain, Pretax Amount	(313)	219	(300)
Total Unrealized Gains (Losses) for Postretirement Obligations, Before Tax	(313)	219	(298)
Total other comprehensive (loss) income, Pretax Amount	24,752	(33,598)	(1,151)
Unrealized holding (losses) gains on securities arising during the period, Tax (Expense) Benefit	(8,803)	12,233	238
Less: reclassification adjustment for (gains) losses on securities included in net income, Tax (Expense) Benefit	193	(405)	67
Total Unrealized (Losses) Gains on Securities, Tax (Expense) Benefit	(8,610)	11,828	305
Transition obligation, Tax (Expense) Benefit	0	0	(1)
Net (loss) gain, Tax (Expense) Benefit	109	(77)	105
Total Unrealized Gains (Losses) for Postretirement Obligations, Tax (Expense) Benefit	109	(77)	104
Total other comprehensive (loss) income, Tax (Expense) Benefit	(8,663)	11,751	409
Unrealized holding (losses) gains on securities arising during the period, Net of Tax Amount	16,350	(22,742)	(423)
Less: reclassification adjustment for (gains) losses on securities included in net income, Net of Tax Amount	(357)	753	(125)
Total Unrealized (Losses) Gains on Securities, Net of Tax	15,993	(21,989)	(548)
Transition obligation, Net of Tax Amount	0	0	1
Net (loss) gain, Net of Tax Amount	(204)	142	(195)
Total Unrealized Gains (Losses) for Postretirement Obligations, Net of Tax	(204)	142	(194)
Total other comprehensive (loss) Income	$ 16,089	$ (21,847)	$ (742)