Retirement Plan Benefits - Pension Plan Investments at Fair Value Represented 5% or More of Pension Plan Net Assets (Details) - Pension Plan Net Assets - Credit Concentration Risk - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	$ 393	$ 421
EB Daily Valued Small Cap Stock Index Fund
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	26	30
EB Daily Valued Stock Index Fund
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	91	103
EB Daily Valued International Stock Index Fund
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	26	27
EB Long Term Government Bond Index
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	49	50
EB Long Term Credit Bond Index
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	71	74
Custom Long Duration Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	$ 130	$ 137