Summary of Significant Accounting Policies Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Change in amounts due for unfulfilled performance obligations	$ (544,780)		$ (567,911)	$ 0
Deferred Costs			281,700
Pension Cost (Reversal of Cost)		$ 74		221
Pension and Other Postretirement Benefits Cost (Reversal of Cost)		$ 221		$ 663
Retained Earnings [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative Effect on Retained Earnings, Net of Tax			172,200
Deferred Charges And Other Assets [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income			268,000
Deferred Revenue [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income			38,000
Deferred Tax Liability [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income			$ 57,800