Revenue from Contracts with Customers (Balance Sheet Impact of the Adoption of ASC606) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
ASSETS
Prepaid expenses and other	$ 9,419		$ 6,233
Other assets	28,703	$ 27,028	16,230
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of other long-term liabilities	14,150	15,750	13,002
Other long-term liabilities	101,410	109,940	100,203
Deferred income taxes	164,964	133,064	133,477
Retained earnings	830,680	798,128	$ 799,402
Mid-Stream [Member] | Adjustments due to ASC606 [Member]
ASSETS
Prepaid expenses and other	206
Other assets	12,383	10,798
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of other long-term liabilities	2,874	2,748
Other long-term liabilities	7,731	9,737
Deferred income taxes	486	(413)
Retained earnings	1,498
Impact of Restatement on Opening Retained Earnings, Net of Tax		$ (1,274)
Mid-Stream [Member] | Amounts without the Adoption of ASC606 [Member]
ASSETS
Prepaid expenses and other	9,213
Other assets	16,320
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of other long-term liabilities	11,276
Other long-term liabilities	93,679
Deferred income taxes	164,478
Retained earnings	$ 829,182