Summary of Significant Accounting Policies Narrative (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Change in amounts due for unfulfilled performance obligations		$ (551,092)		$ (574,737)	$ 0
Deferred Costs	$ 281,200			281,200
Recognized Deferred Sales Costs	$ 6,700			14,000
Pension Cost (Reversal of Cost)			$ 73		147
Pension and Other Postretirement Benefits Cost (Reversal of Cost)			$ 221		$ 442
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