Derivatives (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended			12 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Dec. 31, 2015	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2016	Dec. 31, 2014
Derivative [Line Items]
Payments for (Proceeds from) Derivative Instrument, Operating Activities		$ 302
Gains on cash flow hedge contracts in AOCI to be reclassified into earnings within 12 months					$ 137
Margin posted due to fair value position of hedge contracts	$ 134				134			$ 925
Fuel hedge contracts
Derivative [Line Items]
Derivative gain (loss)	$ 98		$ 97		$ (313)	$ 175
Scenario, Forecast
Derivative [Line Items]
Payments for (Proceeds from) Derivative Instrument, Financing Activities				$ (300)
Payments for (proceeds from) hedging activities							$ 300