Summary of Significant Accounting Policies - Contract Balances (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Changes in the Company's contract assets and contract liabilities
Contract asset, Beginning balance	$ 500	$ 0
Contract asset, Additions	0	0
Contract asset, Deductions	500	5,000
Contract Asset, Impact Of Netting.		5,000
Contract asset, Ending balance	0	0
Contract liabilities:
Contract liabilities, Beginning balance	80,802	89,967
Contract liabilities, Additions	65,287	0
Contract liabilities, Deductions	(158)	(13,196)
Contract Liabilities, Impact Of Netting		5,000
Contract liabilities, Ending balance	$ 145,931	$ 81,771