Summary of Significant Accounting Policies - Assets measured at fair value on nonrecurring basis (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Assets at fair value and gains (losses) recorded
Equipment held for sale	$ 21,054		$ 18,114
Total losses on assets	(8,580)	$ (2,928)
Total losses on equipment held for sale	$ (8,580)	$ (2,928)
Level 1
Assets at fair value and gains (losses) recorded
Equipment held for sale
Assets at fair value
Nonrecurring
Assets at fair value and gains (losses) recorded
Equipment held for sale	$ 10,189	$ 20,795
Assets at fair value	10,189	20,795
Nonrecurring | Level 2
Assets at fair value and gains (losses) recorded
Equipment held for sale	7,524	13,666
Assets at fair value	7,524	13,666
Nonrecurring | Level 3
Assets at fair value and gains (losses) recorded
Equipment held for sale	2,665	7,129
Assets at fair value	$ 2,665	$ 7,129