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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Security gains, net										$ 9,076	$ 333	$ 21
Income tax provision (benefit)		$ (17,455)	$ (15,140)	$ 2,247	$ (1,288)	$ (14,199)	$ (15,902)	$ (10,091)	$ (6,260)	(31,636)	(46,452)	(36,055)
Income before taxes		77,196	65,963	(10,058)	11,855	59,623	69,497	46,060	19,190	144,956	194,370	170,798
Net income attributable to WSFS		$ 59,813	$ 51,145	$ (7,111)	$ 10,927	$ 45,704	$ 53,882	$ 36,200	$ 13,023	114,774	148,809	134,743
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										(7,453)	(668)	(519)
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)										75	33	(10)
Prior service costs (credits)										(266)	(76)	2
Actuarial gains										(47)	(63)	(45)
Income before taxes										(313)	(139)	(43)
Net income attributable to WSFS										(238)	(106)	(53)
Reclassification out of Accumulated Other Comprehensive Income | Defined benefit pension plan settlement
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax provision (benefit)										(67)	0	0
Actuarial gains										279	0	0
Net income attributable to WSFS										212	0	0
Reclassification out of Accumulated Other Comprehensive Income | Net change in fair value of derivatives used for cash flow hedges (1)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax provision (benefit)										107	0	0
Amortization of net unrealized gains to income during the period										(444)	0	0
Net income attributable to WSFS										(337)	0	0
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										(9,076)	(333)	(21)
Income tax provision (benefit)										2,178	80	5
Net income attributable to WSFS										(6,898)	(253)	(16)
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)										59	98	136
Amortization of net unrealized gains to income during the period										(251)	(407)	(586)
Net income attributable to WSFS										$ (192)	$ (309)	$ (450)