Leases (Bareboat and Time Charters-In) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Leases [Line Items]
Total operating lease liabilities	$ 33,993
2019, prior to ASC 842 adoption		$ 12,934
Net minimum lease payments, prior to ASC 842 adoption		$ 12,934
2019, operating days		1421 days
Operating days, total		1421 days
Bareboat Charters-In [Member]
Leases [Line Items]
2019	1,582
2020	6,295
2021	6,278
2022	6,278
2023	4,532
Total lease payments	24,965
less imputed interest	(3,320)
Total operating lease liabilities	$ 21,645
2019, prior to ASC 842 adoption		$ 6,278
2020, prior to ASC 842 adoption		6,295
2021, prior to ASC 842 adoption		6,278
2022, prior to ASC 842 adoption		6,278
2023, prior to ASC 842 adoption		4,782
Net minimum lease payments, prior to ASC 842 adoption		$ 29,911
2019, operating days	184 days	730 days
2020, operating days	732 days	732 days
2021, operating days	730 days	730 days
2022, operating days	730 days	730 days
2023, operating days	556 days	556 days
Operating days, total	2932 days	3478 days
Time Charters-In [Member]
Leases [Line Items]
2019	$ 2,398
2020	5,335
2021	2,170
Total lease payments	9,903
less imputed interest	(472)
Total operating lease liabilities	$ 9,431
2019, operating days	460 days
2020, operating days	1087 days
2021, operating days	408 days
Operating days, total	1955 days
Time Charters In Excluded From Operating Liability [Member]
Leases [Line Items]
2019, prior to ASC 842 adoption	$ 5,530
Lightering Services Component [Member] | Time Charters-In [Member]
Leases [Line Items]
2019, prior to ASC 842 adoption		$ 3,615