Recently Adopted and Recently Issued Accounting Standards - Condensed Consolidated Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 34,651	$ 13,796	$ 96,939	$ 43,664
Adjustments to reconcile net income to net cash used by operating activities:
Deferred income tax expense			1,996	2,018
Changes in assets and liabilities:
Accounts receivable			(29,022)	(26,633)
Inventories			(18,852)	(30,112)
Contract assets			(10,427)	0
Advances on contracts			(12,339)	(6,534)
Other assets and liabilities			(14,164)	8,200
Net Cash Provided by (Used in) Operating Activities			95,014	$ 82,905
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	(2,777)		(1,707)
Adjustments to reconcile net income to net cash used by operating activities:
Deferred income tax expense			(919)
Changes in assets and liabilities:
Accounts receivable			8
Inventories			(34,923)
Contract assets			10,427
Advances on contracts			21,794
Other assets and liabilities			5,320
Net Cash Provided by (Used in) Operating Activities			0
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 31,874		95,232
Adjustments to reconcile net income to net cash used by operating activities:
Deferred income tax expense			1,077
Changes in assets and liabilities:
Accounts receivable			(29,014)
Inventories			(53,775)
Contract assets			0
Advances on contracts			9,455
Other assets and liabilities			(8,844)
Net Cash Provided by (Used in) Operating Activities			$ 95,014