CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Accumulated depreciation	$ 517,742	$ 478,874
Accumulated amortization	$ 77,815	$ 68,648
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