RECENT ACCOUNTING PRONOUNCEMENTS Revenue Recognition - Effect on Statement of Consolidated Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues	$ 2,158,061	$ 1,840,897	$ 6,150,866	$ 5,365,908
Selling, general and administrative expenses	215,506	212,249	637,282	614,809
Earnings from continuing operations before income taxes	116,140	94,492	262,523	235,968
Provision for income taxes	26,629	35,452	95,561	86,997
Earnings from continuing operations	89,511	59,040	166,962	148,971
Net earnings	88,751	58,750	164,514	148,024
Comprehensive income	$ 100,571	$ 92,156	$ 156,435	$ 235,242
Earnings per common share - Basic
Continuing operations (in dollars per share)	$ 1.70	$ 1.12	$ 3.18	$ 2.82
Earnings per common share - Diluted
Continuing operations (in dollars per share)	$ 1.69	$ 1.12	$ 3.15	$ 2.80
Correction of prior period error		$ 8,000		$ 24,000
Charges related to professional fees		4,000		4,000
Service
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues	$ 1,106,107	888,613	$ 3,106,575	2,595,141
Cost of services	$ 945,851	753,838	$ 2,639,137	2,186,316
As Previously Reported | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues		1,848,500		5,389,900
Selling, general and administrative expenses		216,700		620,000
Earnings from continuing operations before income taxes		94,300		235,000
Provision for income taxes		35,400		86,500
Earnings from continuing operations		58,900		148,500
Net earnings		58,600		147,600
Comprehensive income		$ 92,000		$ 234,800
Earnings per common share - Basic
Continuing operations (in dollars per share)		$ 1.12		$ 2.81
Earnings per common share - Diluted
Continuing operations (in dollars per share)		$ 1.11		$ 2.79
As Previously Reported | Service | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues		$ 896,200		$ 2,619,100
Cost of services		761,500		2,210,300
New Revenue Standard Adjustment | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues		(7,600)		(24,000)
Selling, general and administrative expenses		(4,400)		(5,200)
Earnings from continuing operations before income taxes		100		1,000
Provision for income taxes		0		500
Earnings from continuing operations		100		400
Net earnings		100		400
Comprehensive income		$ 100		$ 400
Earnings per common share - Basic
Continuing operations (in dollars per share)		$ 0.00		$ 0.01
Earnings per common share - Diluted
Continuing operations (in dollars per share)		$ 0.01		$ 0.01
New Revenue Standard Adjustment | Service | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues		$ (7,600)		$ (24,000)
Cost of services		$ (7,600)		$ (24,000)