Change in Accumulated Other Comprehensive Income - Components of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Security gains, net										$ 333	$ 21	$ 1,984
Income tax provision (benefit)		$ (14,199)	$ (15,902)	$ (10,091)	$ (6,260)	$ (8,486)	$ (9,893)	$ (6,907)	$ (10,769)	(46,452)	(36,055)	(58,246)
Interest income		137,350	141,262	142,903	99,577	77,794	75,415	72,151	67,613	521,092	292,973	254,726
Income before taxes		59,623	69,497	46,060	19,190	38,204	48,828	35,647	48,119	194,370	170,798	108,490
Net income attributable to WSFS		$ 45,704	$ 53,882	$ 36,200	$ 13,023	$ 29,718	$ 38,935	$ 28,740	$ 37,350	148,809	134,743	50,244
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 attributable to WSFS										(668)	(519)	(1,764)
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)										33	(10)	56
Prior service costs (credits)										(76)	2	(76)
Actuarial losses										(63)	(45)	(70)
Income before taxes										(139)	(43)	(146)
Net income attributable to WSFS										(106)	(53)	(90)
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										(333)	(21)	(1,984)
Income tax provision (benefit)										80	5	704
Net income attributable to WSFS										(253)	(16)	(1,280)
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)										98	136	241
Interest income										(407)	(586)	(635)
Net income attributable to WSFS										$ (309)	$ (450)	$ (394)