Basis of Condensed Consolidated Financial Statements (Impact of Adoption of New Accounting Standard on Statement of Operations) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenues	$ 488,401	$ 473,978	$ 933,301	$ 913,829
Cost of services	239,346	249,162	478,735	501,128
Selling, general and administrative expenses	112,022	103,552	226,974	215,400
Operating income	89,637	78,393	134,056	110,958
Provision for income taxes	17,307	18,635	16,596	24,909
Net income	58,516	$ 41,260	86,803	$ 56,698
Accounting Standards Update 2014-09 | Effect of Change Higher/(Lower)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenues	24,079		24,574
Cost of services	(317)		(454)
Selling, general and administrative expenses	(769)		(197)
Operating income	25,165		25,225
Provision for income taxes	5,699		5,690
Net income	19,466		19,535
Accounting Standards Update 2014-09 | Balances Without Adoption Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenues	464,322		908,727
Cost of services	239,663		479,189
Selling, general and administrative expenses	112,791		227,171
Operating income	64,472		108,831
Provision for income taxes	11,608		10,906
Net income	$ 39,050		$ 67,268