Summary of Significant Accounting Policies - Contract Balances (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Changes in the Company's contract assets and contract liabilities
Contract asset, Beginning balance	$ 500,000
Contract asset, Additions	(500,000)	$ (4,041,000)
Contract asset, Deductions	500,000
Contract asset, Ending balance		4,041,000
Contract liabilities:
Contract liabilities, Beginning balance	80,802,000	89,967,000
Contract liabilities, Additions	65,287,000
Contract liabilities, Deductions	(79,000)	(7,834,000)
Contract liabilities, Ending balance	$ 146,010,000	$ 82,133,000