Supplemental Condensed Consolidating Guarantor and Non-Guarantor Financial Information - Condensed Consolidating Statement Of Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues	$ 467,711	$ 448,962	$ 444,715	$ 446,344	$ 441,920	$ 438,042	$ 460,147	$ 470,285	$ 1,807,732	$ 1,810,394	$ 1,779,149
Operating expenses
Direct cost of revenues	307,566	294,851	304,071	309,072	308,239	293,702	303,194	305,636	1,215,560	1,210,771	1,171,444
Selling, general and administrative expenses	111,176	103,909	107,342	107,295	116,478	106,220	108,245	103,609	429,722	434,552	432,668
Special charges	10,811	0	30,074	0	3,634	0	1,750	5,061	40,885	10,445
Acquisition-related contingent consideration	867	252	777	395	623	201	206	1,134	2,291	2,164	(1,200)
Amortization of other intangible assets	2,766	2,882	2,422	2,493	2,265	2,845	2,590	2,606	10,563	10,306	11,726
Costs and Expenses	433,186	401,894	444,686	419,255	431,239	402,968	415,985	418,046	1,699,021	1,668,238	1,614,638
Operating income	34,525	47,068	29	27,089	10,681	35,074	44,162	52,239	108,711	142,156	164,511
Other income (expense)									(21,606)	(14,353)	(59,125)
Income before income tax provision (benefit)	28,430	41,411	(4,629)	21,893	5,269	31,983	41,984	48,567	87,105	127,803	105,386
Income tax provision (benefit)	(38,458)	9,197	527	7,877	(1,832)	10,292	15,437	18,386	(20,857)	42,283	39,333
Equity in net earnings of subsidiaries									0	0	0
Net income	$ 66,888	$ 32,214	$ (5,156)	$ 14,016	$ 7,101	$ 21,691	$ 26,547	$ 30,181	107,962	85,520	66,053
Other comprehensive loss, net of tax:
Foreign currency translation adjustments, net of tax expense of $0									30,664	(41,884)	(28,727)
Other comprehensive loss, net of tax									30,664	(41,884)	(28,727)
Comprehensive income									138,626	43,636	37,326
Foreign currency translation adjustments, tax expense									0	0	0
Eliminations
Revenues									(9,416)	(9,721)	(146,997)
Operating expenses
Direct cost of revenues									(9,206)	(9,461)	(146,597)
Selling, general and administrative expenses									(210)	(260)	(399)
Special charges									0	0
Acquisition-related contingent consideration									0	0	0
Amortization of other intangible assets									(2,989)	(3,084)	(3,521)
Costs and Expenses									(12,405)	(12,805)	(150,517)
Operating income									2,989	3,084	3,520
Other income (expense)									0	0	0
Income before income tax provision (benefit)									2,989	3,084	3,520
Income tax provision (benefit)									0	0	0
Equity in net earnings of subsidiaries									(173,296)	(132,599)	(108,647)
Net income									(170,307)	(129,515)	(105,127)
Other comprehensive loss, net of tax:
Foreign currency translation adjustments, net of tax expense of $0									0	0	0
Other comprehensive loss, net of tax									0	0	0
Comprehensive income									(170,307)	(129,515)	(105,127)
FTI Consulting
Revenues									652,604	671,408	667,259
Operating expenses
Direct cost of revenues									438,395	447,254	428,356
Selling, general and administrative expenses									181,713	190,546	189,607
Special charges									15,414	2,916
Acquisition-related contingent consideration									279	6	(1,408)
Amortization of other intangible assets									4,393	3,903	3,944
Costs and Expenses									640,194	644,625	620,499
Operating income									12,410	26,783	46,760
Other income (expense)									(23,684)	(27,228)	(64,554)
Income before income tax provision (benefit)									(11,274)	(445)	(17,794)
Income tax provision (benefit)									11,070	1,222	(6,944)
Equity in net earnings of subsidiaries									130,306	87,187	76,903
Net income									107,962	85,520	66,053
Other comprehensive loss, net of tax:
Foreign currency translation adjustments, net of tax expense of $0									0	0	0
Other comprehensive loss, net of tax									0	0	0
Comprehensive income									107,962	85,520	66,053
Guarantor Subsidiaries
Revenues									603,294	625,950	754,458
Operating expenses
Direct cost of revenues									421,826	428,158	551,829
Selling, general and administrative expenses									125,552	124,019	121,112
Special charges									13,010	6,242
Acquisition-related contingent consideration									2,012	2,158	208
Amortization of other intangible assets									2,141	2,179	2,861
Costs and Expenses									564,541	562,756	676,010
Operating income									38,753	63,194	78,448
Other income (expense)									(5,932)	(2,811)	(4,881)
Income before income tax provision (benefit)									32,821	60,383	73,567
Income tax provision (benefit)									(43,846)	27,961	35,579
Equity in net earnings of subsidiaries									42,990	45,412	31,744
Net income									119,657	77,834	69,732
Other comprehensive loss, net of tax:
Foreign currency translation adjustments, net of tax expense of $0									0	0	0
Other comprehensive loss, net of tax									0	0	0
Comprehensive income									119,657	77,834	69,732
Non-Guarantor Subsidiaries
Revenues									561,250	522,757	504,429
Operating expenses
Direct cost of revenues									364,545	344,820	337,856
Selling, general and administrative expenses									122,667	120,247	122,348
Special charges									12,461	1,287
Acquisition-related contingent consideration									0	0	0
Amortization of other intangible assets									7,018	7,308	8,442
Costs and Expenses									506,691	473,662	468,646
Operating income									54,559	49,095	35,783
Other income (expense)									8,010	15,686	10,310
Income before income tax provision (benefit)									62,569	64,781	46,093
Income tax provision (benefit)									11,919	13,100	10,698
Equity in net earnings of subsidiaries									0	0	0
Net income									50,650	51,681	35,395
Other comprehensive loss, net of tax:
Foreign currency translation adjustments, net of tax expense of $0									30,664	(41,884)	(28,727)
Other comprehensive loss, net of tax									30,664	(41,884)	(28,727)
Comprehensive income									$ 81,314	$ 9,797	$ 6,668