Basis of Condensed Consolidated Financial Statements (Impact of Adoption of New Accounting Standard on Statement of Operations) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenues	$ 451,768	$ 483,131	$ 1,385,069	$ 1,396,960
Cost of services	230,419	244,186	709,154	745,314
Selling, general and administrative expenses	113,075	131,323	340,049	346,723
Operating income	59,780	62,296	193,836	173,255
Provision for income taxes	20,836	11,851	37,432	36,759
Net income	22,451	$ 30,754	109,254	$ 87,453
Accounting Standards Update 2014-09 | Effect of Change Higher/(Lower)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenues	529		25,103
Cost of services	58		(396)
Selling, general and administrative expenses	(739)		(936)
Operating income	1,210		26,435
Provision for income taxes	302		5,992
Net income	908		20,443
Accounting Standards Update 2014-09 | Balances Without Adoption Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenues	451,239		1,359,966
Cost of services	230,361		709,550
Selling, general and administrative expenses	113,814		340,985
Operating income	58,570		167,401
Provision for income taxes	20,534		31,440
Net income	$ 21,543		$ 88,811