Schedule of Company's Aggregate Credit Facility Commitments, Letter of Credit Sub-Limit Under Revolving Credit Facility and Commercial Paper Limit and Available Capacity (Parenthetical) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Revolving Credit Facility
Short Term Debt [Line Items]
Letters of credit, outstanding amount	$ 82	$ 38