Leases - Lease Revenue (Details)	3 Months Ended	9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)
Lessor, Lease, Description [Line Items]
Lessor, Sales-type Lease, Renewal Term	50 years	50 years
Lessor, Sales-type Lease, Term of Contract	30 years	30 years
Residual Value of Leased Asset	$ 3,900,000	$ 3,900,000
Lease Revenue [Abstract]
Operating leases	48,000	143,000
Sales-type leases	4,895,000	7,039,000
Total lease revenue	4,943,000	7,182,000
Operating Leases Payment to be Received [Abstract]
Operating Lease, Payments to be Received, Remainder of Fiscal Year	18,000	18,000
Operating Lease, Payments to be Received, Two Years	61,000	61,000
Operating Lease, Payments to be Received, Three Years	49,000	49,000
Operating Lease, Payments to be Received, Four Years	45,000	45,000
Operating Lease, Payments to be Received, Five Years	45,000	45,000
Operating Lease, Payments to be Received, Thereafter	138,000	138,000
Total lease revenue	356,000	356,000
Total Lease Payments to be Received [Abstract]
Lease Payments to be Received, Remainder of Fiscal Year	4,859,000	4,859,000
Lease Payments to be Received, Two Years	18,290,000	18,290,000
Lease Payments to be Received, Three Years	17,567,000	17,567,000
Lease Payments to be Received, Four Years	17,071,000	17,071,000
Lease Payments to be Received, Five Years	16,602,000	16,602,000
Lease Payments to be Received, Thereafter	281,075,000	281,075,000
Total lease revenue	$ 355,464,000	$ 355,464,000
Compressed Natural Gas Lease [Member]
Lessor, Lease, Description [Line Items]
Lessor, Sales-type Lease, Term of Contract	10 years	10 years
Residual Value of Leased Asset	$ 0	$ 0
Variable Lease, Payment		0
North Mist Lease [Member]
Lessor, Lease, Description [Line Items]
Residual Value of Leased Asset	0	0
Variable Lease, Payment		0
Sales-type Lease [Member]
Sales-type Leases Payments to be Received [Abstract]
Sales-type Lease, Payments to be Received, Remainder of Fiscal Year	4,841,000	4,841,000
Sales-type Lease, Payments to be Received, Two Years	18,229,000	18,229,000
Sales-type Lease, Payments to be Received, Three Years	17,518,000	17,518,000
Sales-type Lease, Payments to be Received, Four Years	17,026,000	17,026,000
Sales-type Lease, Payments to be Received, Five Years	16,557,000	16,557,000
Sales-type Lease, Payments to be Received, Thereafter	280,937,000	280,937,000
Total lease revenue	355,108,000	355,108,000
Less: imputed interest	205,596,000	205,596,000
Total leases receivable	$ 149,512,000	$ 149,512,000