Income Taxes (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Loss Carryforwards [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%		35.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount		$ (307,101)
Unrecognized Tax Benefits	$ 53,752	$ 53,752	$ 22,137	$ 0	$ 0
Tax Credit Carryforward, Expiration Date		16 years
Unrecognized Tax Benefits, Interest on Income Taxes Accrued		$ 31,615	$ 22,137	$ 0
Effective Income Tax Rate Reconciliation, Tax Contingency, Domestic, Percent	25.00%
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount	$ 315,000
Tax Year 2011 [Member]
Operating Loss Carryforwards [Line Items]
Income Tax Examination, Year under Examination		2012
Tax Year 2013 [Member]
Operating Loss Carryforwards [Line Items]
Income Tax Examination, Year under Examination		2013
OKLAHOMA
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforward, Amount	9,700	$ 9,700
KANSAS
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforward, Amount	$ 16,800	$ 16,800