Comprehensive Income - Components of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Beginning Balance, Defined Benefit Pension Plan	$ (22,776)	$ (7,966)
Defined Benefit Pension Plan, Other comprehensive income before reclassifications	0	0
Defined Benefit Pension Plan, Amounts reclassified from accumulated other comprehensive income	475	236
Defined Benefit Pension Plan, Period change	475	236
Ending Balance, Defined Benefit Pension Plan	(22,301)	(7,730)
Beginning Balance, Unrealized Gains (Losses) on Securities Available-for-Sale	2,892	(6,126)
Unrealized Gains (Losses) on Securities Available-for-Sale, Other comprehensive income before reclassifications	4,804	3,075
Unrealized Gains (Losses) on Securities Available-for-Sale, Amounts reclassified from accumulated other comprehensive income	(11)	49
Net effect on accumulated other comprehensive income for the period	4,793	3,124
Ending Balance, Unrealized Gains (Losses) on Securities Available-for-Sale	7,685	(3,002)
Beginning Balance, Unrealized Gains on Securities Transferred from Available-for-Sale to Held-to-Maturity	1,059	1,358
Unrealized Gains on Securities Transferred from Available-for-Sale to Held-to-Maturity, Other comprehensive income before reclassifications	0	0
Unrealized Gains on Securities Transferred from Available-for-Sale to Held-to-Maturity, Amounts reclassified from accumulated other comprehensive income	(67)	(87)
Unrealized Gains on Securities Transferred from Available-for-Sale to Held-to-Maturity, Period change	(67)	(87)
Ending Balance, Unrealized Gains on Securities Transferred from Available-for-Sale to Held-to-Maturity	992	1,271
Beginning Balance, Accumulated Other Comprehensive Income	(18,825)	(12,734)
Other comprehensive income before reclassifications	4,804	3,075
Amounts reclassified from accumulated other comprehensive income	397	198
Total other comprehensive income	5,201	3,273
Ending Balance, Accumulated Other Comprehensive Income	$ (13,624)	$ (9,461)