Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 2,593,921	$ 1,978,827	$ 1,395,321
Amounts reclassified from accumulated other comprehensive income/(loss)	325	1,970	1,949
Total other comprehensive gain (loss)	30,158	39,072	(5,313)
Ending Balance	2,756,737	2,593,921	1,978,827
Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(26,591)
Ending Balance		(26,591)
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(17,468)	(16,542)	(18,626)
Other comprehensive income/(loss) before reclassifications	(320)	(3,239)	(4,277)
Acquired FFKT post-retirement medical benefit plan			4,235
Amounts reclassified from accumulated other comprehensive income/(loss)	2,286	2,313	2,126
Total other comprehensive gain (loss)	1,966	(926)	2,084
Ending Balance	(15,502)	(17,468)	(16,542)
Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	18,644	(21,522)	(13,250)
Other comprehensive income/(loss) before reclassifications	30,153	40,341	(7,220)
Amounts reclassified from accumulated other comprehensive income/(loss)	(1,936)	(175)	11
Total other comprehensive gain (loss)	28,217	40,166	(7,209)
Ending Balance	46,861	18,644	(21,522)
Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member] | ASU 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(1,063)
Ending Balance			(1,063)
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	25	193	381
Amounts reclassified from accumulated other comprehensive income/(loss)	(25)	(168)	(188)
Total other comprehensive gain (loss)	(25)	(168)	(188)
Ending Balance		25	193
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	1,201	(37,871)	(31,495)
Other comprehensive income/(loss) before reclassifications	29,833	37,102	(11,497)
Acquired FFKT post-retirement medical benefit plan			4,235
Amounts reclassified from accumulated other comprehensive income/(loss)	325	1,970	1,949
Total other comprehensive gain (loss)	30,158	39,072	(5,313)
Ending Balance	$ 31,359	1,201	(37,871)
Accumulated Other Comprehensive Income (Loss) [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(1,063)
Accumulated Other Comprehensive Income (Loss) [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member] | ASU 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		$ (1,063)
Ending Balance			$ (1,063)