Summary of Significant Accounting Policies Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Change in amounts due for unfulfilled performance obligations	$ (606,888)	$ 0
Deferred Costs	274,700
Recognized Deferred Sales Costs	7,300
Pension Cost (Reversal of Cost)		74
Pension and Other Postretirement Benefits Cost (Reversal of Cost)		$ 221
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