Other Assets and Accrued Liabilities - Asset Removal Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	$ 21,703	$ 21,507	$ 7,266
Liabilities acquired in acquisition	0	0	14,056
Additional liabilities accrued	3,357	2,404	157
Changes to prior estimates	3,504	(1,695)	0
Payments	(443)	(1,296)	(609)
Accretion expense	1,045	783	637
Balance at end of year	$ 29,166	$ 21,703	$ 21,507