Revenue Recognition - Contract Balances (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Receivables, which are included in accounts receivable - trade, net	$ 1,532,000		$ 1,532,000		$ 2,129,000
Contract assets, net, which are included in other current assets	5,296,000		5,296,000		8,766,000
Payables, which are included in other current liabilities	1,084,000		1,084,000		1,663,000
Contract liabilities, which are included in other current liabilities	7,830,000		7,830,000		9,358,000
Contract assets, net, which are included in other assets	25,957,000		25,957,000		18,443,000
Contract liabilities, which are included in other liabilities	32,125,000		32,125,000		$ 29,045,000
Revenue recognized on contract liabilities	3,921,000		6,658,000
Revenue recognized relating to performance obligations satisfied or partially satisfied in prior periods	$ 0	$ 0	$ 0	$ 0