Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Asset retirement obligation [Roll Forward]
Balance at beginning of year	$ 375,553	$ 341,969
Liabilities incurred	25,869	13,424
Liabilities acquired	486	1,002
Liabilities settled	(7,097)	(3,699)
Accretion expense	19,789	18,242
Revisions in estimates	2,975	4,615
Balance at end of year	417,575	375,553
Asset retirement obligation costs
Regulatory Assets [Line Items]
Accretion expense, including asset retirement obligations	$ 18,300	$ 16,800