CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Accumulated depreciation	$ 603,196	$ 566,081
Accumulated amortization	$ 92,546	$ 87,471
Voting units
Members' equity units, authorized (in units)	100	100
Members' equity units issued (in units)	100	100
Members' equity units outstanding (in units)	100	100
Non-voting units
Members' equity units, authorized (in units)	100	100
Members' equity units issued (in units)	100	100
Members' equity units outstanding (in units)	100	100