CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Accumulated depreciation	$ 493,699	$ 477,909
Accumulated amortization	$ 73,231	$ 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