Revenue Recognition - Initial Impact of New Revenue Recognition Standard (Details) - USD ($) $ in Thousands	Mar. 31, 2018	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
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
Accounting Standards Update 2014-09 | Calculated under Revenue Guidance in Effect before Topic 606
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