Description of Business and Summary of Significant Accounting Policies - Changes in Liability for Asset Removal Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	$ 7,266	$ 6,928	$ 6,485
Liabilities acquired in acquisition	14,056	0	0
Additional liabilities accrued	157	490	403
Changes to prior estimates	0	(467)	0
Payments made	(609)	(77)	(334)
Accretion expense	637	392	374
Balance at end of year	$ 21,507	$ 7,266	$ 6,928