Income Taxes - Narrative (Details) - USD ($) $ in Thousands			3 Months Ended				12 Months Ended
	Dec. 22, 2017	Dec. 21, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary Of Income Taxes [Line Items]
Expected provision at federal statutory tax rate	21.00%	35.00%					21.00%	35.00%	35.00%
Reduction in net deferred tax liabilities			$ 196,000
Alternative minimum tax							$ 1,100
Valuation allowance			454				254	$ 454
Foreign tax credits							8,600
State net operating losses							1,400
Excess tax (deficiency) benefit from equity-based compensation							0	0	$ (470)
Income tax deficiency			$ (165,231)	$ 34,506	$ 45,391	$ 19,498	49,227	(65,836)	153,774
Domestic Tax Authority
Summary Of Income Taxes [Line Items]
Operating Loss Carryforwards							703,000
State and Local Jurisdiction
Summary Of Income Taxes [Line Items]
Operating Loss Carryforwards							114,500
Accounting Standards Update 2016-09
Summary Of Income Taxes [Line Items]
Income tax deficiency							$ 400	$ 500	$ (500)