Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income/(Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 1,122,132	$ 788,190	$ 746,595
Total other comprehensive loss	(6,172)	(2,129)	(6,091)
Ending Balance	1,341,408	1,122,132	788,190
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(17,539)	(22,776)	(7,966)
Other comprehensive (loss)/income before reclassifications	(2,112)	3,233	(15,768)
Amounts reclassified from accumulated other comprehensive income/(loss)	1,893	2,004	958
Total other comprehensive loss	(219)	5,237	(14,810)
Ending Balance	(17,758)	(17,539)	(22,776)
Accumulated Unrealized Gains (Losses) on Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(4,162)	2,892	(6,126)
Other comprehensive (loss)/income before reclassifications	(4,300)	(6,677)	9,638
Amounts reclassified from accumulated other comprehensive income/(loss)	(1,428)	(377)	(620)
Total other comprehensive loss	(5,728)	(7,054)	9,018
Ending Balance	(9,890)	(4,162)	2,892
Accumulated Unrealized Gains on Securities Transferred from Available For Sale to Held to Maturity [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	747	1,059	1,358
Amounts reclassified from accumulated other comprehensive income/(loss)	(225)	(312)	(299)
Total other comprehensive loss	(225)	(312)	(299)
Ending Balance	522	747	1,059
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(20,954)	(18,825)	(12,734)
Other comprehensive (loss)/income before reclassifications	(6,412)	(3,444)	(6,130)
Amounts reclassified from accumulated other comprehensive income/(loss)	240	1,315	39
Total other comprehensive loss	(6,172)	(2,129)	(6,091)
Ending Balance	$ (27,126)	$ (20,954)	$ (18,825)