Recent Accounting Pronouncements Revenue Recognition - Effect on Statement of Consolidated 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
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 2,258,349	$ 2,158,061	$ 2,089,338	$ 1,903,467	$ 1,931,146	$ 1,840,897	$ 1,788,028	$ 1,736,983	$ 8,409,215	$ 7,297,054	$ 6,758,138
Cost of services										2,970,800	2,574,100
Selling, general and administrative expenses									854,807	871,224	804,229
Earnings from continuing operations before income taxes	111,338	116,140	98,283	48,100	78,577	94,492	80,854	60,622	373,861	314,545	407,256
Provision for income taxes									98,254	(477,744)	142,024
Earnings from continuing operations	108,645	89,511	43,519	33,932	643,318	59,040	51,395	38,536	275,607	792,289	265,232
Net earnings	$ 108,784	$ 88,751	$ 42,258	$ 33,505	$ 643,808	$ 58,750	$ 50,868	$ 38,406	273,298	791,832	263,069
Comprehensive income									$ 170,047	$ 918,363	$ 141,772
Earnings per common share - Basic
Continuing operations (in dollars per share)	$ 2.07	$ 1.70	$ 0.83	$ 0.65	$ 12.22	$ 1.12	$ 0.97	$ 0.73	$ 5.24	$ 15.00	$ 4.99
Earnings per common share - Diluted
Continuing operations (in dollars per share)	$ 2.06	$ 1.69	$ 0.82	$ 0.64	$ 12.12	$ 1.12	$ 0.97	$ 0.72	$ 5.21	$ 14.90	$ 4.95
Correction of prior period error										$ 33,000	$ 29,000
Services revenue
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue									$ 4,280,834	3,538,869	3,123,448
Cost of services									$ 3,655,794	2,970,803	2,574,132
As Previously Reported | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues										7,329,600	6,787,000
Selling, general and administrative expenses										872,000	805,100
Earnings from continuing operations before income taxes										313,800	406,400
Provision for income taxes										(477,200)	141,700
Earnings from continuing operations										791,000	264,600
Net earnings										790,600	262,500
Comprehensive income										$ 917,100	$ 141,200
Earnings per common share - Basic
Continuing operations (in dollars per share)										$ 14.98	$ 4.98
Earnings per common share - Diluted
Continuing operations (in dollars per share)										$ 14.87	$ 4.94
As Previously Reported | Services revenue | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue										$ 3,571,400	$ 3,152,300
Cost of services										3,003,300	2,603,000
Adjustment | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues										(32,500)	(28,900)
Selling, general and administrative expenses										(800)	(900)
Earnings from continuing operations before income taxes										800	900
Provision for income taxes										(500)	300
Earnings from continuing operations										1,300	600
Net earnings										1,300	600
Comprehensive income										$ 1,300	$ 600
Earnings per common share - Basic
Continuing operations (in dollars per share)										$ 0.02	$ 0.01
Earnings per common share - Diluted
Continuing operations (in dollars per share)										$ 0.03	$ 0.01
Adjustment | Services revenue | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue										$ (32,500)	$ (28,900)
Cost of services										$ (32,500)	$ (28,900)