Basis of Condensed Consolidated Financial Statements (Impact of Adoption of New Accounting Standard on Statement of Operations) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenues	$ 444,900	$ 439,851
Cost of services	239,389	251,966
Selling, general and administrative expenses	114,952	111,850
Operating income	44,419	32,563
Benefit for income taxes	(711)	6,274
Net income	28,287	$ 15,438
Accounting Standards Update 2014-09 | Effect of Change Higher/(Lower)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenues	495
Cost of services	(137)
Selling, general and administrative expenses	572
Operating income	60
Benefit for income taxes	(9)
Net income	69
Accounting Standards Update 2014-09 | Balances Without Adoption Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenues	444,405
Cost of services	239,526
Selling, general and administrative expenses	114,380
Operating income	44,359
Benefit for income taxes	(702)
Net income	$ 28,218