Summary of Significant Accounting Policies - Capital Leased Assets (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Capital Leased Assets [Line Items]
Capital Leased Assets, Accumulated Amortization	$ (59)	$ (49)
Capital Leased Assets, Net of Amortization	152	161
Office Building [Member]
Capital Leased Assets [Line Items]
Capital Leased Assets, Gross	61	61
Nitrogen Plant [Member]
Capital Leased Assets [Line Items]
Capital Leased Assets, Gross	83	83
Computer-Related Equipment [Member]
Capital Leased Assets [Line Items]
Capital Leased Assets, Gross	61	60
Gas Pipeline [Member]
Capital Leased Assets [Line Items]
Capital Leased Assets, Gross	$ 6	$ 6