Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 1,978,827	$ 1,395,321	$ 1,341,408
Amounts reclassified from accumulated other comprehensive income/(loss)	1,970	1,949	1,958
Total other comprehensive gain (loss)	39,072	(5,313)	1,212
Ending Balance	2,593,921	1,978,827	1,395,321
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(16,542)	(18,626)	(17,758)
Other comprehensive income/(loss) before reclassifications	(3,239)	(4,277)	239
Acquired FFKT post-retirement medical benefit plan		4,235
Amounts reclassified from accumulated other comprehensive income/(loss)	2,313	2,126	2,194
Total other comprehensive gain (loss)	(926)	2,084	2,433
Adoption of Accounting Standard ASU 2016-01			(3,301)
Ending Balance	(17,468)	(16,542)	(18,626)
Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(21,522)	(13,250)	(9,890)
Other comprehensive income/(loss) before reclassifications	40,341	(7,220)	(985)
Amounts reclassified from accumulated other comprehensive income/(loss)	(175)	11	(27)
Total other comprehensive gain (loss)	40,166	(7,209)	(1,012)
Adoption of Accounting Standard ASU 2016-01		(1,063)	(2,348)
Ending Balance	18,644	(21,522)	(13,250)
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	193	381	522
Amounts reclassified from accumulated other comprehensive income/(loss)	(168)	(188)	(209)
Total other comprehensive gain (loss)	(168)	(188)	(209)
Adoption of Accounting Standard ASU 2016-01			68
Ending Balance	25	193	381
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(37,871)	(31,495)	(27,126)
Other comprehensive income/(loss) before reclassifications	37,102	(11,497)	(746)
Acquired FFKT post-retirement medical benefit plan		4,235
Amounts reclassified from accumulated other comprehensive income/(loss)	1,970	1,949	1,958
Total other comprehensive gain (loss)	39,072	(5,313)	1,212
Adoption of Accounting Standard ASU 2016-01		(1,063)	(5,581)
Ending Balance	$ 1,201	$ (37,871)	$ (31,495)