Revenue Recognition - Historical Method (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Assets
Contracts in progress	$ 337,375			$ 420,628
Deferred Income Taxes	84,727			86,740
Liabilities
Accrued liabilities – other	51,662			64,738
Advance billings on contracts	77,140			246,192
Stockholders' Equity
Retained earnings	1,053,090			990,652
Revenues	457,463	$ 428,229
Cost of operations	327,364	303,216
Operating Income	79,888	76,272
Provision for Income Taxes	18,603	24,592
Net Income	66,413	$ 55,786
Accounting Standards Update 2014-09
Assets
Contracts in progress	337,375		$ 260,932
Deferred Income Taxes	84,727		85,193
Liabilities
Accrued liabilities – other	51,662		66,371
Advance billings on contracts	77,140		73,390
Stockholders' Equity
Retained earnings	1,053,090		$ 1,000,578
Revenues	457,463
Cost of operations	327,364
Operating Income	79,888
Provision for Income Taxes	18,603
Net Income	66,413
Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09
Assets
Contracts in progress	365,192			420,628
Deferred Income Taxes	85,443			86,740
Liabilities
Accrued liabilities – other	49,327			64,738
Advance billings on contracts	117,174			246,192
Stockholders' Equity
Retained earnings	1,043,924			$ 990,652
Revenues	465,530
Cost of operations	334,542
Operating Income	80,777
Provision for Income Taxes	18,732
Net Income	$ 67,173