Long Term Debt - Narrative 2 (Details)		3 Months Ended	6 Months Ended
	Sep. 17, 2012 USD ($) engine	Jun. 30, 2015 USD ($) engine	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Long Term Debt
Net proceeds received from notes issued and sold			$ 118,700,000	$ 10,000,000
Total notes payable		$ 860,979,000	860,979,000		$ 840,956,000
WEST II Series 2012-A term notes payable at a fixed rate of interest maturing in September 2037. Secured by engines | WEST II
Long Term Debt
Face amount	$ 390,000,000
Net proceeds received from notes issued and sold	$ 384,900,000
Debt Instrument Expected Payoff Period		10 years
Total notes payable		$ 327,800,000	$ 327,800,000		$ 351,900,000
Number of engines pledged as collateral | engine	22	65
Fixed amortization of notes payable		$ 0
Fixed rate (as a percent)		5.50%	5.50%
Number of business days to pay interest		5 days
Period of time over which maintenance obligations are projected		6 months
WEST II Series 2012-A term notes payable at a fixed rate of interest maturing in September 2037. Secured by engines | WEST II | Minimum
Long Term Debt
Minimum amount of cash from maintenance reserve payments required to be held in restricted cash account		$ 9,000,000	$ 9,000,000
Prior WEST notes
Long Term Debt
Total notes payable	$ 435,900,000