Summary of Significant Accounting Policies - Schedule Of Cumulative Effects Change (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Assets
Trade accounts receivable and contract assets	$ 217,481		$ 189,737	$ 196,840
Other current assets	17,199		92,077	91,544
Other noncurrent assets, net	252,085		225,359	202,814
Assets	6,965,235		6,873,325	6,857,350
Liabilities
Contract liabilities	74,135		64,417	65,959
Accrued expenses and other	66,728		77,567	77,312
Deferred Income Tax Liabilities, Net	479,671		442,753	439,631
Other noncurrent liabilities	104,868		103,559	107,627
Liabilities	4,587,091		4,555,873	4,558,106
Equity [Abstract]
Accumulated other comprehensive loss	(91,077)		(52,389)	(52,822)
Accumulated earnings	689,160		600,458	582,683
Total shareholders’ equity	2,378,144	$ 2,357,597	2,317,452	2,299,244	$ 2,049,032	$ 2,027,867	$ 1,755,023
Liabilities and Equity	6,965,235		6,873,325	6,857,350
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Assets
Trade accounts receivable and contract assets				196,840
Other current assets				91,544
Other noncurrent assets, net				202,814
Assets				6,857,350
Liabilities
Contract liabilities				65,959
Accrued expenses and other				77,312
Deferred Income Tax Liabilities, Net				439,631
Other noncurrent liabilities				107,627
Liabilities				4,558,106
Equity [Abstract]
Total shareholders’ equity				2,299,244
Liabilities and Equity				6,857,350
Calculated Under Revenue Guidance In Effect Before Topic 606 And Accounting Standards Update 2016-01 [Member]
Equity [Abstract]
Accumulated other comprehensive loss				(52,822)
Accumulated earnings				$ 582,683
Accounting Standards Update 2014-09 [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Assets
Trade accounts receivable and contract assets	225,992
Other current assets	16,666
Other noncurrent assets, net	218,775
Assets	6,939,903
Liabilities
Contract liabilities	75,106
Accrued expenses and other	66,473
Other noncurrent liabilities	107,102
Liabilities	4,585,139
Equity [Abstract]
Total shareholders’ equity	2,354,764
Liabilities and Equity	6,939,903
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Assets
Trade accounts receivable and contract assets	8,511		(7,103)
Other current assets	(533)		533
Other noncurrent assets, net	(33,310)		22,545
Assets	(25,332)		15,975
Liabilities
Contract liabilities	971		(1,542)
Accrued expenses and other	(255)		255
Deferred Income Tax Liabilities, Net			3,122
Other noncurrent liabilities	2,234		(4,068)
Liabilities	(1,952)		(2,233)
Equity [Abstract]
Accumulated other comprehensive loss			0
Accumulated earnings	(23,380)		18,208
Total shareholders’ equity	(23,380)		18,208
Liabilities and Equity	(25,332)		15,975
Accounting Standards Update 2016-01 [Member]
Equity [Abstract]
Accumulated other comprehensive loss	650		433
Accumulated earnings	$ (650)		(433)
Accumulated Earnings | Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			$ 18,200