CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Accumulated depreciation	$ 395,871	$ 374,738
Accumulated amortization	$ 54,896	$ 50,313
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