EBP Plan Assets (Details 4) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Maximum percentage of total stock portfolio invested in the common stock of any one corporation	5.00%
Maximum percentage of portfolio invested in fixed income securities of any one issuer with exception of bond index funds and U. S. government guaranteed and agency securities	5.00%
Each investment manager responsibility to manage the plans' funds, minimum	1.00%
Each investment manager responsibility to manage the plans' funds, maximum	28.00%
Fair value, plan assets, Level 1 to Level 2 transfers, amount	$ 0	$ 0
Fair value, plan assets, Level 2 to Level 1 transfers, amount	$ 0	0
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Notice Period	30 days
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Notice Period	10 days
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of active investment managers managing plan funds	12
Number of passive investment managers managing plan funds	1
Fair value, plan assets, Level 1 to Level 2 transfers, amount	$ 0
Fair value, plan assets, Level 2 to Level 1 transfers, amount	0
Fair values of plan assets
Total assets at fair value	$ 1,241	$ 1,293	$ 1,241
Pension Benefits [Member] | Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan asset target allocation	60.00%
Pension Benefits [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan asset target allocation	40.00%
Weighted average duration of fixed income security portfolio	8 years	6 years
Pension Benefits [Member] | Plan assets within fair value hierarchy [Member]
Fair values of plan assets
Total assets at fair value	$ 465	$ 770
Pension Benefits [Member] | Cash management fund [Member]
Fair values of plan assets
Total assets at fair value	8	25
Pension Benefits [Member] | U.S. large cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	83	221
Pension Benefits [Member] | U.S. small cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	64	139
Pension Benefits [Member] | International developed markets large cap growth [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value		60
Pension Benefits [Member] | U.S. Treasury securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	65	31
Pension Benefits [Member] | Government and municipal bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	8
Pension Benefits [Member] | Mortgage and asset-backed securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	87	65
Pension Benefits [Member] | Corporate bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	145	222
Pension Benefits [Member] | Insurance company investment contracts and other [Member]
Fair values of plan assets
Total assets at fair value	5	7
Pension Benefits [Member] | Equities - U.S. large cap [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value	367	189
Pension Benefits [Member] | Equities - International small cap [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value	27	24
Pension Benefits [Member] | Equities - International emerging markets [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value	50
Pension Benefits [Member] | Equities - International developed markets [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value	153
Pension Benefits [Member] | Equities - Emerging markets value [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value		27
Pension Benefits [Member] | Equities - Emerging markets growth [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value		19
Pension Benefits [Member] | Equities - International developed markets large cap value [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value		101
Pension Benefits [Member] | Fixed income - U.S. long duration [Member] | Commingled investment funds - fixed income [Member]
Fair values of plan assets
Total assets at fair value	95
Pension Benefits [Member] | Fixed income - Corporate bonds [Member] | Commingled investment funds - fixed income [Member]
Fair values of plan assets
Total assets at fair value	84	163
Pension Benefits [Member] | Level 1 [Member]
Fair values of plan assets
Total assets at fair value	220	416
Pension Benefits [Member] | Level 1 [Member] | Cash management fund [Member]
Fair values of plan assets
Total assets at fair value	8	25
Pension Benefits [Member] | Level 1 [Member] | U.S. large cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	83	221
Pension Benefits [Member] | Level 1 [Member] | U.S. small cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	64	139
Pension Benefits [Member] | Level 1 [Member] | International developed markets large cap growth [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value		0
Pension Benefits [Member] | Level 1 [Member] | U.S. Treasury securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	65	31
Pension Benefits [Member] | Level 1 [Member] | Government and municipal bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0
Pension Benefits [Member] | Level 1 [Member] | Mortgage and asset-backed securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 1 [Member] | Corporate bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 1 [Member] | Insurance company investment contracts and other [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 2 [Member]
Fair values of plan assets
Total assets at fair value	245	354
Pension Benefits [Member] | Level 2 [Member] | Cash management fund [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 2 [Member] | U.S. large cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 2 [Member] | U.S. small cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 2 [Member] | International developed markets large cap growth [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value		60
Pension Benefits [Member] | Level 2 [Member] | U.S. Treasury securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 2 [Member] | Government and municipal bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	8
Pension Benefits [Member] | Level 2 [Member] | Mortgage and asset-backed securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	87	65
Pension Benefits [Member] | Level 2 [Member] | Corporate bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	145	222
Pension Benefits [Member] | Level 2 [Member] | Insurance company investment contracts and other [Member]
Fair values of plan assets
Total assets at fair value	5	7
Pension Benefits [Member] | Level 3 [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 3 [Member] | Cash management fund [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 3 [Member] | U.S. large cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 3 [Member] | U.S. small cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 3 [Member] | International developed markets large cap growth [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value		0
Pension Benefits [Member] | Level 3 [Member] | U.S. Treasury securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 3 [Member] | Government and municipal bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0
Pension Benefits [Member] | Level 3 [Member] | Mortgage and asset-backed securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 3 [Member] | Corporate bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Pension Benefits [Member] | Level 3 [Member] | Insurance company investment contracts and other [Member]
Fair values of plan assets
Total assets at fair value	$ 0	0
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Proportion of assets subjected to income tax	38.00%
Number of active investment managers managing plan funds	4
Number of passive investment managers managing plan funds	1
Fair values of plan assets
Total assets at fair value	$ 201	$ 208	$ 201
Other Postretirement Benefits [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted average duration of fixed income security portfolio	7 years	5 years
Other Postretirement Benefits [Member] | Plan assets within fair value hierarchy [Member]
Fair values of plan assets
Total assets at fair value	$ 122	$ 156
Other Postretirement Benefits [Member] | Cash management fund [Member]
Fair values of plan assets
Total assets at fair value	11	13
Other Postretirement Benefits [Member] | U.S. large cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	37	53
Other Postretirement Benefits [Member] | U.S. small cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	20	28
Other Postretirement Benefits [Member] | International developed markets large cap growth [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	10	15
Other Postretirement Benefits [Member] | Emerging markets growth [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	1	3
Other Postretirement Benefits [Member] | U.S. Treasury securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	7	3
Other Postretirement Benefits [Member] | Government and municipal bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	12	11
Other Postretirement Benefits [Member] | Mortgage and asset-backed securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	9	7
Other Postretirement Benefits [Member] | Corporate bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	15	23
Other Postretirement Benefits [Member] | Equities - U.S. large cap [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value	37	19
Other Postretirement Benefits [Member] | Equities - International small cap [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value	3	2
Other Postretirement Benefits [Member] | Equities - International emerging markets [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value	5
Other Postretirement Benefits [Member] | Equities - International developed markets [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value	16
Other Postretirement Benefits [Member] | Equities - Emerging markets value [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value		3
Other Postretirement Benefits [Member] | Equities - Emerging markets growth [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value		2
Other Postretirement Benefits [Member] | Equities - International developed markets large cap value [Member] | Commingled investment funds - equities [Member]
Fair values of plan assets
Total assets at fair value		10
Other Postretirement Benefits [Member] | Fixed income - U.S. long duration [Member] | Commingled investment funds - fixed income [Member]
Fair values of plan assets
Total assets at fair value	10
Other Postretirement Benefits [Member] | Fixed income - Corporate bonds [Member] | Commingled investment funds - fixed income [Member]
Fair values of plan assets
Total assets at fair value	8	16
Other Postretirement Benefits [Member] | Level 1 [Member]
Fair values of plan assets
Total assets at fair value	76	98
Other Postretirement Benefits [Member] | Level 1 [Member] | Cash management fund [Member]
Fair values of plan assets
Total assets at fair value	11	13
Other Postretirement Benefits [Member] | Level 1 [Member] | U.S. large cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	37	53
Other Postretirement Benefits [Member] | Level 1 [Member] | U.S. small cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	20	28
Other Postretirement Benefits [Member] | Level 1 [Member] | International developed markets large cap growth [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	1	0
Other Postretirement Benefits [Member] | Level 1 [Member] | Emerging markets growth [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	0	1
Other Postretirement Benefits [Member] | Level 1 [Member] | U.S. Treasury securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	7	3
Other Postretirement Benefits [Member] | Level 1 [Member] | Government and municipal bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 1 [Member] | Mortgage and asset-backed securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 1 [Member] | Corporate bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 2 [Member]
Fair values of plan assets
Total assets at fair value	46	58
Other Postretirement Benefits [Member] | Level 2 [Member] | Cash management fund [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 2 [Member] | U.S. large cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 2 [Member] | U.S. small cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 2 [Member] | International developed markets large cap growth [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	9	15
Other Postretirement Benefits [Member] | Level 2 [Member] | Emerging markets growth [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	1	2
Other Postretirement Benefits [Member] | Level 2 [Member] | U.S. Treasury securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 2 [Member] | Government and municipal bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	12	11
Other Postretirement Benefits [Member] | Level 2 [Member] | Mortgage and asset-backed securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	9	7
Other Postretirement Benefits [Member] | Level 2 [Member] | Corporate bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	15	23
Other Postretirement Benefits [Member] | Level 3 [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 3 [Member] | Cash management fund [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 3 [Member] | U.S. large cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 3 [Member] | U.S. small cap [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 3 [Member] | International developed markets large cap growth [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 3 [Member] | Emerging markets growth [Member] | Equity securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 3 [Member] | U.S. Treasury securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 3 [Member] | Government and municipal bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 3 [Member] | Mortgage and asset-backed securities [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	0	0
Other Postretirement Benefits [Member] | Level 3 [Member] | Corporate bonds [Member] | Fixed income securities [Member]
Fair values of plan assets
Total assets at fair value	$ 0	$ 0