Leases (Present Value of Lease Liabilities) (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Leases [Abstract]
Operating Leases, Total future minimum payments	$ 905,574	[1]
Operating Leases, Less imputed interest	(281,613)	[1]
Operating Leases, Present value of lease liabilities	623,961	[1],[2]
Finance Leases, Total future minimum payments	10,687
Finance Leases, Less imputed interest	(2,234)
Finance Leases, Present value of lease liabilities	8,453	[2]
Total Leases, Future minimum payments	916,261
Total Leases, Less imputed interest	(283,847)
Total Leases, Present value of lease liabilities	$ 632,414	[2]

[1] Excludes $272.2 million of minimum lease payments for leases entered but not yet commenced. These payments relate to an expansion of an existing gas processing plant and payments are anticipated to commence at the end of 2019 for 20 years.

[2] Includes both the current and long-term portion of the lease liabilities.