Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			$ 1,395,321	$ 1,341,408
Amounts reclassified from accumulated other comprehensive income	$ 538	$ 470	926	1,076
Total other comprehensive income			(20,794)	5,008
Ending Balance	1,524,106	1,377,537	1,524,106	1,377,537
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(18,626)	(17,758)
Other comprehensive income before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income	589	509	1,026	1,164
Total other comprehensive income			1,026	1,164
Adoption of Accounting Standard ASU 2016-01	0		0
Ending Balance	(17,600)	(16,594)	(17,600)	(16,594)
Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(13,250)	(9,890)
Other comprehensive income before reclassifications			(21,720)	3,932
Amounts reclassified from accumulated other comprehensive income		35	0	35
Total other comprehensive income			(21,720)	3,967
Adoption of Accounting Standard ASU 2016-01	(1,063)		(1,063)
Ending Balance	(36,033)	(5,923)	(36,033)	(5,923)
Accumulated Unrealized Gains on Debt Securities Transferred from Available For Sale to Held to Maturity [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			381	522
Other comprehensive income before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income	(51)	(74)	(100)	(123)
Total other comprehensive income			(100)	(123)
Adoption of Accounting Standard ASU 2016-01	0		0
Ending Balance	281	399	281	399
Accumulated Other Comprehensive (Loss) Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(31,495)	(27,126)
Other comprehensive income before reclassifications			(21,720)	3,932
Amounts reclassified from accumulated other comprehensive income			926	1,076
Total other comprehensive income			(20,794)	5,008
Adoption of Accounting Standard ASU 2016-01	(1,063)		(1,063)
Ending Balance	$ (53,352)	$ (22,118)	$ (53,352)	$ (22,118)