Derivatives and Hedge Accounting - Location of Gain or Loss Recognized (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	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
Derivative Instruments, Gain (Loss) [Line Items]
Cost of revenues									$ 584,855	[1],[2]	$ 495,142	[1],[2]	$ 447,718
General & administrative expenses									116,202	[1]	102,515	[1]	88,616
Selling & marketing expenses									63,612	[1]	53,379	[1]	50,579
Depreciation & amortization									48,566		38,549	[1]	34,580
Foreign exchange gain/(loss), net									4,787		2,839	[1]	5,597	[1]
Net income attributable to ExlService Holdings, Inc. stockholders	$ 3,857	$ 15,249	$ 14,462	$ 23,158	$ (9,355)	$ 21,077	$ 20,378	$ 16,788	56,726		48,888	[1],[3]	61,733	[1],[3]
Reclassification out of Accumulated Other Comprehensive Income | Derivatives Designated as Hedging Instruments | Derivatives in cash flow hedging relationships | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Cost of revenues									2,481		5,465		0
General & administrative expenses									443		960		0
Selling & marketing expenses									44		103		0
Depreciation & amortization									181		371		0
Foreign exchange gain/(loss), net									0		0		2,669
Net income attributable to ExlService Holdings, Inc. stockholders									3,149		6,899		2,669
Reclassification out of Accumulated Other Comprehensive Income | Derivatives Designated as Hedging Instruments | Fair value hedge | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange gain/(loss), net									(3,224)		5,056		4,790
Reclassification out of Accumulated Other Comprehensive Income | Derivative not designated as hedging instruments | Fair value hedge | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange gain/(loss), net									$ (3,224)		$ 5,056		$ 4,790

[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.