Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Assets measured at fair value
Short-term investments	$ 253,300	$ 0
Recurring basis
Assets measured at fair value
Cash equivalents	155,967	269,384
Restricted cash	6,566	21,000
Short-term investments	253,300
Total assets measured at fair value	429,035	317,586
Liabilities measured at fair value
Total liabilities measured at fair value	20,187	21,547
Recurring basis | Fuel derivative contracts | Crude oil call options
Assets measured at fair value
Derivative contracts	50	7,121
Liabilities measured at fair value
Derivative contracts	50	7,121
Recurring basis | Fuel derivative contracts | Crude oil put options
Assets measured at fair value
Derivative contracts	4	186
Liabilities measured at fair value
Derivative contracts	4	186
Recurring basis | Fuel derivative contracts | Heating oil put options
Assets measured at fair value
Derivative contracts	845	417
Recurring basis | Fuel derivative contracts | Heating oil swaps
Assets measured at fair value
Derivative contracts		5,863
Liabilities measured at fair value
Derivative contracts	19,515	187
Recurring basis | Foreign currency derivatives
Assets measured at fair value
Derivative contracts	11,851	12,494
Liabilities measured at fair value
Derivative contracts	118	1,188
Recurring basis | Interest rate derivative
Assets measured at fair value
Derivative contracts	452	1,121
Recurring basis | Negative interest arbitrage derivative
Liabilities measured at fair value
Derivative contracts	500	12,865
Recurring basis | Level 1
Assets measured at fair value
Cash equivalents	152,967	269,384
Restricted cash	6,566	21,000
Total assets measured at fair value	159,533	290,384
Recurring basis | Level 2
Assets measured at fair value
Cash equivalents	3,000
Short-term investments	253,300
Total assets measured at fair value	269,502	27,202
Liabilities measured at fair value
Total liabilities measured at fair value	19,687	8,682
Recurring basis | Level 2 | Fuel derivative contracts | Crude oil call options
Assets measured at fair value
Derivative contracts	50	7,121
Liabilities measured at fair value
Derivative contracts	50	7,121
Recurring basis | Level 2 | Fuel derivative contracts | Crude oil put options
Assets measured at fair value
Derivative contracts	4	186
Liabilities measured at fair value
Derivative contracts	4	186
Recurring basis | Level 2 | Fuel derivative contracts | Heating oil put options
Assets measured at fair value
Derivative contracts	845	417
Recurring basis | Level 2 | Fuel derivative contracts | Heating oil swaps
Assets measured at fair value
Derivative contracts		5,863
Liabilities measured at fair value
Derivative contracts	19,515	187
Recurring basis | Level 2 | Foreign currency derivatives
Assets measured at fair value
Derivative contracts	11,851	12,494
Liabilities measured at fair value
Derivative contracts	118	1,188
Recurring basis | Level 2 | Interest rate derivative
Assets measured at fair value
Derivative contracts	452	1,121
Recurring basis | Level 3
Liabilities measured at fair value
Total liabilities measured at fair value	500	12,865
Recurring basis | Level 3 | Negative interest arbitrage derivative
Liabilities measured at fair value
Derivative contracts	$ 500	$ 12,865