Joint Ventures (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended	15 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2017	Sep. 30, 2017	Jan. 04, 2016
Schedule of Equity Method Investments [Line Items]
Current portion of deferred income recorded in connection with joint venture		$ 6,439			$ 6,439	$ 6,451
Noncurrent portion of deferred income recorded in connection with joint venture		235,855			$ 235,855	236,896
Equity interest in earnings (losses) of joint venture		$ 596	$ 684
Woodward and General Electric Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Agreement to Form Joint Venture, Execution Date				Jan. 04, 2016
Equity Method Investment, Ownership Percentage		50.00%			50.00%
Cash received from formation of joint venture				$ 250,000
Amount of deferred income recorded in connection with joint venture							$ 250,000
Current portion of deferred income recorded in connection with joint venture		$ 6,439			$ 6,439	6,451
Noncurrent portion of deferred income recorded in connection with joint venture		235,855			235,855	236,896
Equity interest in earnings (losses) of joint venture		596	684
Cash distributions from joint venture					0
Net investment in joint venture		6,868			6,868	6,272
Accounts receivable from related party		6,728			6,728	8,554
Accounts payable to related party		3,984			$ 3,984	$ 6,741
Woodward and General Electric Joint Venture [Member] | Sales [Member]
Schedule of Equity Method Investments [Line Items]
Amortization of deferred income recognized as an increase to sales		1,053	1,496
Sales to related party		12,975	15,302
Reduction to sales related to royalties paid to joint venture		$ 5,408	$ 5,403
Woodward and General Electric Joint Venture [Member] | Scenario, Forecast [Member]
Schedule of Equity Method Investments [Line Items]
Cash received annually from formation of joint venture	$ 4,894