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Employee Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2008
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Minimum participant age to elect						40 years
Employer discretionary contribution amount			$ 14	$ 12	$ 11
Requisite service period			3 years
Actual return on plan assets			16.60%	0.60%
Total recognized in net periodic benefit cost and other comprehensive income			$ (7)	$ 21	31
Amortization of actuarial loss			2
Amortization of prior service cost			1
Pension plan assets at fair value			$ 315	278	$ 288
Domestic Equity Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments			83.00%
Domestic Equity Securities \| Information Technology Sector
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments			22.00%
Domestic Equity Securities \| Financial Services Sector [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments			20.00%
Domestic Equity Securities \| Healthcare Sector
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments			14.00%
Domestic Equity Securities \| Consumer Staple Sector [Member] [Domain]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments			10.00%
Domestic Equity Securities \| Consumer Discretionary Sector [Member] [Domain]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments			10.00%
Domestic Equity Securities \| Industrial Sector
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments			10.00%
Domestic Equity Securities \| All Remaining Sectors
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments			10.00%
States, Municipalities and Political Subdivisions
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments			10.00%
Domestic Corporate Debt Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments			7.00%
Cash and Cash Equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Pension plan assets at fair value			$ 11	$ 21
Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Matching contribution to defined contribution plan			6.00%
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Pension plan assets, shares			232,113	267,113
Pension plan assets, fair value			$ 18	$ 16
Shares sold during period			35,000	200,000
Cash dividends paid		$ 1	$ 1	$ 1
Pension Plan \| Benefit Obligations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Percentage of increase decrease in discount rate			0.25%
Pension Plan \| Net Periodic Benefit Costs
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Percentage of increase decrease in discount rate			0.30%
Supplemental Employee Retirement Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Accrued employee benefit			$ 15	14
Expected contribution by employer during 2016			$ 12
Supplemental Employee Retirement Plan \| Benefit Obligations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Percentage of increase decrease in discount rate			0.20%
Supplemental Employee Retirement Plan \| Net Periodic Benefit Costs
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Percentage of increase decrease in discount rate			0.25%
Top Hat Savings Plan - Mutual Funds and Common Equities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
CFC Top Hat Savings Plan			$ 24	$ 21
Subsequent Event \| Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Expected contribution by employer during 2016	$ 5