Summary of Significant Accounting Policies (Details 4) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Assets at fair value and gains (losses) recorded
Equipment held for sale	$ 31,506,000		$ 31,506,000		$ 23,607,000
Total losses on equipment held for sale			(6,268,000)		(5,874,000)
Total losses on assets			(6,268,000)		(5,874,000)
Asset write-down	4,283,000	2,474,000	6,268,000	2,756,000
Nonrecurring
Assets at fair value and gains (losses) recorded
Asset write-down recorded earlier			3,500,000		4,700,000
Asset write-down			2,800,000		1,200,000
Nonrecurring | Total
Assets at fair value and gains (losses) recorded
Equipment held for sale	31,506,000		31,506,000		23,607,000
Assets at fair value	31,506,000		31,506,000		23,607,000
Nonrecurring | Level 2
Assets at fair value and gains (losses) recorded
Equipment held for sale	29,839,000		29,839,000		23,384,000
Assets at fair value	29,839,000		29,839,000		23,384,000
Nonrecurring | Level 3
Assets at fair value and gains (losses) recorded
Equipment held for sale	1,667,000		1,667,000		223,000
Assets at fair value	$ 1,667,000		$ 1,667,000		$ 223,000