Change in Accumulated Other Comprehensive Income - Components of Other Comprehensive Income (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Security gains, net										$ 21	$ 1,984	$ 2,369
Income tax provision (benefit)		$ (8,486)	$ (9,893)	$ (6,907)	$ (10,769)	$ (27,864)	$ (10,942)	$ (10,850)	$ (8,590)	(36,055)	(58,246)	(33,074)
Income before taxes		38,204	48,828	35,647	48,119	18,032	31,511	31,420	27,527	170,798	108,490	97,154
Net income		$ 29,718	$ 38,935	$ 28,740	$ 37,350	$ (9,832)	$ 20,569	$ 20,570	$ 18,937	134,743	50,244	64,080
Prior service cost (credit), tax adjustment	$ 100
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income										(519)	(1,764)	(1,703)
Reclassification out of Accumulated Other Comprehensive Income | Net change in defined benefit plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax provision (benefit)										(10)	56	(103)
Prior service costs (credits)										2	(76)	(76)
Transition obligation										0	0	0
Actuarial losses										(45)	(70)	348
Income before taxes										(43)	(146)	272
Net income										(53)	(90)	169
Reclassification out of Accumulated Other Comprehensive Income | Realized gains on securities transactions | Net change in investment securities available for sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Security gains, net										(21)	(1,984)	(2,369)
Income tax provision (benefit)										5	704	900
Net income										(16)	(1,280)	(1,469)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of net unrealized gains to income during the period | Net change in investment securities available for sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax provision (benefit)										136	241	248
Interest and dividends on investment securities										(586)	(635)	(651)
Net income										$ (450)	$ (394)	$ (403)