|**Employee Retirement Benefits (Tables)**             | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                          
|----------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Retirement Benefits [Abstract]**                    |                                                                                                                                                                                                                                                                                                                                                                                                         
|Schedule of Defined Benefit Plans Disclosures         | The following summarizes the components of net periodic benefit cost for our qualified and supplemental pension plans:(Dollars in millions)Three months ended March 31,20202019Service cost$2$2Non-service costs (benefit):Interest cost33Expected return on plan assets(5)(5)Amortization of actuarial loss and prior service cost1—Other—1Total non-service benefit(1)(1)Net periodic benefit cost$1$1
|                                                     
|                                                      |                                                                                                                                                                                                                                                                                                                                                                                                         
|(Dollars in millions)                                 |                                                                                                                                                                                                                                                                                                                                                                                                         
|                                                      |                                                                                                                                                                                                                                                                                                                                                                                                         
|Service cost                                          |                                                                                                                                                                                                                                                                                                                                                                                                         
|Non-service costs (benefit):                          |                                                                                                                                                                                                                                                                                                                                                                                                         
|Interest cost                                         |                                                                                                                                                                                                                                                                                                                                                                                                         
|Expected return on plan assets                        |                                                                                                                                                                                                                                                                                                                                                                                                         
|Amortization of actuarial loss and prior service cost |                                                                                                                                                                                                                                                                                                                                                                                                         
|Other                                                 |                                                                                                                                                                                                                                                                                                                                                                                                         
|Total non-service benefit                             |                                                                                                                                                                                                                                                                                                                                                                                                         
|Net periodic benefit cost                             |                                                                                                                                                                                                                                                                                                                                                                                                         
|                                                      |                                                                                                                                                                                                                                                                                                                                                                                                         

X- ReferencesNo definition available.
+ Details**Name:**
us-gaap_CompensationAndRetirementDisclosureAbstract**Namespace Prefix:**
us-gaap_**Data Type:**
xbrli:stringItemType**Balance Type:**
na**Period Type:**
duration X- DefinitionTabular disclosure of one or more of the entity's defined benefit pension plans or one or more other defined benefit postretirement plans, separately for pension plans and other postretirement benefit plans including the entity's schedule of fair value of plan assets for defined benefit or other postretirement plans.
+ ReferencesReference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2439-114920Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2410-114920Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2417-114920
+ Details**Name:**
us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTextBlock**Namespace Prefix:**
us-gaap_**Data Type:**
nonnum:textBlockItemType**Balance Type:**
na**Period Type:**
duration