Revenue from Contracts with Customers - Schedule of Consolidated Balance Sheet and Statement of Operations (Details) - USD ($) $ / shares in Units, $ 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	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 363,907				$ 467,430				$ 363,907	$ 467,430
Accounts receivable	1,773,620				1,607,922				1,773,620	1,607,922
Contract assets	158,243				122,621				158,243	122,621	$ 130,697
Inventories	42,321				42,724				42,321	42,724
Prepaid expenses and other	48,116				43,812				48,116	43,812
Total current assets	2,386,207				2,284,509				2,386,207	2,284,509
Investments, notes and other long-term receivables	2,899				2,309				2,899	2,309
Property, plant and equipment, net	134,351				127,156				134,351	127,156
Goodwill	990,887				964,893				990,887	964,893	979,628
Identifiable intangible assets, net	488,286				495,036				488,286	495,036
Other assets	86,177				92,001				86,177	92,001
Total assets	4,088,807				3,965,904				4,088,807	3,965,904	3,852,438
Current liabilities:
Current maturities of long-term debt and capital lease obligations	16,013				15,364				16,013	15,364
Accounts payable	652,091				567,840				652,091	567,840
Contract liabilities	552,290				524,156				552,290	524,156	489,242
Accrued payroll and benefits	343,069				322,865				343,069	322,865
Other accrued expenses and liabilities	170,935				220,727				170,935	220,727
Total current liabilities	1,734,398				1,650,952				1,734,398	1,650,952
Borrowings under revolving credit facility	25,000				25,000				25,000	25,000
Long-term debt and capital lease obligations	254,764				269,786				254,764	269,786
Other long-term obligations	333,204				346,049				333,204	346,049
Total liabilities	2,347,366				2,291,787				2,347,366	2,291,787
Total equity	1,741,441				1,674,117				1,741,441	1,674,117	1,537,942	$ 1,480,056
Total liabilities and equity	4,088,807				3,965,904				4,088,807	3,965,904
Income Statement [Abstract]
Revenues	2,229,308	$ 2,047,049	$ 1,953,886	$ 1,900,388	2,012,639	$ 1,886,691	$ 1,895,937	$ 1,891,732	8,130,631	7,686,999	7,551,524
Cost of sales									6,925,178	6,539,987	6,513,662
Gross profit	336,151	309,339	290,844	269,119	311,101	295,070	274,501	266,340	1,205,453	1,147,012	1,037,862
Selling, general and administrative expenses									799,157	758,714	727,067
Restructuring expenses									2,306	1,577	1,438
Impairment loss on goodwill and identifiable intangible assets	0	0	907	0	57,819	0	0	0	907	57,819	2,428
Operating income									403,083	328,902	306,929
Net periodic pension (cost) income									(2,743)	(1,652)	(1,529)
Interest expense									(13,544)	(12,770)	(12,627)
Interest income									2,746	965	663
Income from continuing operations before income taxes									395,028	318,749	296,494
Income tax provision									109,106	90,699	111,199
Income from continuing operations									285,922	228,050	185,295
Loss from discontinued operation, net of income taxes									(2,345)	(857)	(3,142)
Net income including noncontrolling interests									283,577	227,193	182,153
Less: Net (income) loss attributable to noncontrolling interests									(46)	3	(218)
Net income attributable to EMCOR Group, Inc.	$ 77,968	$ 79,373	$ 70,816	$ 55,374	$ 53,201	$ 64,597	$ 56,758	$ 52,640	$ 283,531	$ 227,196	$ 181,935
Basic earnings per common share:
From continuing operations attributable to EMCOR Group, Inc. common stockholders (in US dollars per share)	$ 1.39	$ 1.37	$ 1.22	$ 0.95	$ 0.91	$ 1.10	$ 0.96	$ 0.89	$ 4.92	$ 3.85	$ 3.05
Diluted earnings per common share:
From continuing operations attributable to EMCOR Group, Inc. common stockholders (in US dollars per share)	$ 1.38	$ 1.36	$ 1.21	$ 0.94	$ 0.90	$ 1.09	$ 0.95	$ 0.88	$ 4.89	$ 3.83	$ 3.02
Pro Forma [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Current assets:
Cash and cash equivalents	$ 363,907								$ 363,907
Accounts receivable	1,778,930								1,778,930
Contract assets	145,583								145,583
Inventories	54,173								54,173
Prepaid expenses and other	46,687								46,687
Total current assets	2,389,280								2,389,280
Investments, notes and other long-term receivables	2,899								2,899
Property, plant and equipment, net	134,351								134,351
Goodwill	990,887								990,887
Identifiable intangible assets, net	488,286								488,286
Other assets	86,177								86,177
Total assets	4,091,880								4,091,880
Current liabilities:
Current maturities of long-term debt and capital lease obligations	16,013								16,013
Accounts payable	652,091								652,091
Contract liabilities	511,089								511,089
Accrued payroll and benefits	343,069								343,069
Other accrued expenses and liabilities	219,166								219,166
Total current liabilities	1,741,428								1,741,428
Borrowings under revolving credit facility	25,000								25,000
Long-term debt and capital lease obligations	254,764								254,764
Other long-term obligations	333,204								333,204
Total liabilities	2,354,396								2,354,396
Total equity	1,737,484								1,737,484
Total liabilities and equity	$ 4,091,880								4,091,880
Income Statement [Abstract]
Revenues									8,117,087
Cost of sales									6,918,281
Gross profit									1,198,806
Selling, general and administrative expenses									799,157
Restructuring expenses									2,306
Impairment loss on goodwill and identifiable intangible assets									907
Operating income									396,436
Net periodic pension (cost) income									(2,743)
Interest expense									(13,544)
Interest income									2,746
Income from continuing operations before income taxes									388,381
Income tax provision									107,270
Income from continuing operations									281,111
Loss from discontinued operation, net of income taxes									(2,345)
Net income including noncontrolling interests									278,766
Less: Net (income) loss attributable to noncontrolling interests									(46)
Net income attributable to EMCOR Group, Inc.									$ 278,720
Basic earnings per common share:
From continuing operations attributable to EMCOR Group, Inc. common stockholders (in US dollars per share)									$ 4.84
Diluted earnings per common share:
From continuing operations attributable to EMCOR Group, Inc. common stockholders (in US dollars per share)									$ 4.81