CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Accumulated depreciation	$ 641,125	$ 566,081
Accumulated amortization	$ 95,406	$ 87,471
Voting Units
Members' equity units, authorized (in shares)	100	100
Members' equity units, issued (in shares)	100	100
Members' equity units, outstanding (in shares)	100	100
Non-voting Units
Members' equity units, authorized (in shares)	100	100
Members' equity units, issued (in shares)	100	100
Members' equity units, outstanding (in shares)	100	100