Segment & Geographical Information - Revenues and Cost of Revenues for Company's Reportable Segments (Detail)		1 Months Ended	3 Months Ended								12 Months Ended
		Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) operating_segment		Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($)
Revenues and cost of revenues for Company's reportable segments [Line Items]
Revenues, net			$ 234,903,000	$ 231,124,000	$ 210,112,000	$ 206,973,000	$ 197,875,000	$ 192,345,000	$ 189,057,000	$ 183,033,000	$ 883,112,000		$ 762,310,000	[1]	$ 685,988,000	[1]
Cost of revenues											584,855,000	[1],[2]	495,142,000	[1],[2]	447,718,000
Gross profit			79,955,000	78,967,000	70,463,000	68,872,000	68,616,000	69,268,000	65,323,000	63,961,000	298,257,000	[2]	267,168,000	[1],[2]	238,270,000	[1],[2]
Operating expenses	[1]										248,436,000		194,443,000		173,775,000
Foreign exchange gain, interest expense and other income, net											10,549,000		12,309,000		19,389,000
Income tax expense											3,397,000		36,146,000	[1]	22,151,000	[1]
Loss from equity-method investment		$ 0									247,000		0	[1],[3]	0	[1],[3]
Net income attributable to ExlService Holdings, Inc. stockholders			$ 3,857,000	$ 15,249,000	$ 14,462,000	$ 23,158,000	$ (9,355,000)	$ 21,077,000	$ 20,378,000	$ 16,788,000	$ 56,726,000		48,888,000	[1],[3]	61,733,000	[1],[3]
Number of operating segments, industry focused | operating_segment											5
Number of operating segments, finance and accounting | operating_segment											1
BPM and related services
Revenues and cost of revenues for Company's reportable segments [Line Items]
Revenues, net											$ 597,822,000		552,367,000		520,254,000
Analytics services
Revenues and cost of revenues for Company's reportable segments [Line Items]
Revenues, net											285,290,000		209,943,000		165,734,000
Insurance
Revenues and cost of revenues for Company's reportable segments [Line Items]
Revenues, net											258,144,000		234,794,000		206,327,000
Cost of revenues											174,921,000		159,433,000		146,151,000
Gross profit											83,223,000		75,361,000		60,176,000
Healthcare
Revenues and cost of revenues for Company's reportable segments [Line Items]
Revenues, net											84,391,000		77,013,000		68,656,000
Cost of revenues											66,768,000		49,412,000		44,060,000
Gross profit											17,623,000		27,601,000		24,596,000
TT&L
Revenues and cost of revenues for Company's reportable segments [Line Items]
Revenues, net											70,237,000		70,951,000		69,366,000
Cost of revenues											41,066,000		41,337,000		41,923,000
Gross profit											29,171,000		29,614,000		27,443,000
F&A
Revenues and cost of revenues for Company's reportable segments [Line Items]
Revenues, net											97,941,000		86,527,000		79,416,000
Cost of revenues											59,155,000		51,362,000		48,258,000
Gross profit											38,786,000		35,165,000		31,158,000
All Other
Revenues and cost of revenues for Company's reportable segments [Line Items]
Revenues, net											87,109,000		83,082,000		96,489,000
Cost of revenues											58,341,000		56,638,000		61,019,000
Gross profit											28,768,000		26,444,000		35,470,000
Analytics
Revenues and cost of revenues for Company's reportable segments [Line Items]
Revenues, net											285,290,000		209,943,000		165,734,000
Cost of revenues											184,604,000		136,960,000		106,307,000
Gross profit											$ 100,686,000		$ 72,983,000		$ 59,427,000

[1] Adjusted pursuant to adoption of ASU No. 2017-07, Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post-retirement Benefit Cost. Refer to Note 21 and Note 27 to the consolidated financial statements.

[2] Exclusive of depreciation and amortization.

[3] Adjusted pursuant to adoption of ASU 2016-18, Statements of Cash Flows, Restricted Cash. Refer to Note 8 and Note 27 to the consolidated financial statements.