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Benefit Plans (Tables) | 6 Months Ended
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Jun. 30, 2018
Retirement Benefits [Abstract] |
Company's pension and postretirement benefit plans |
Components of net periodic cost included in operating costs and expenses
|  | Three months ended June 30, |  | Six months ended June 30,
($ in millions) |  | 2018 |  | 2017 |  | 2018 |  | 2017
Pension benefits |  |  |  |  |  |  |  |
Service cost |  | $ | 28 |  |  | $ | 28 |  |  | $ | 56 |  |  | $ | 57 |
Interest cost |  | 61 |  |  | 66 |  |  | 121 |  |  | 132 |
Expected return on plan assets |  | (106 | ) |  | (102 | ) |  | (211 | ) |  | (204 | )
Amortization of: |  |  |  |  |  |  |  |  |  |
Prior service credit |  | (14 | ) |  | (14 | ) |  | (28 | ) |  | (28 | )
Net actuarial loss |  | 45 |  |  | 47 |  |  | 89 |  |  | 94 |
Settlement loss |  | 7 |  |  | 8 |  |  | 14 |  |  | 16 |
Net periodic pension cost |  | $ | 21 |  |  | $ | 33 |  |  | $ | 41 |  |  | $ | 67 |
Postretirement benefits |  |  |  |  |  |  |  |
Service cost |  | $ | 2 |  |  | $ | 2 |  |  | $ | 4 |  |  | $ | 4 |
Interest cost |  | 4 |  |  | 3 |  |  | 7 |  |  | 7 |
Amortization of: |  |  |  |  |  |  |  |
Prior service credit |  | (6 | ) |  | (6 | ) |  | (11 | ) |  | (12 | )
Net actuarial gain |  | (5 | ) |  | (6 | ) |  | (11 | ) |  | (12 | )
Net periodic postretirement credit |  | $ | (5 | ) |  | $ | (7 | ) |  | $ | (11 | ) |  | $ | (13 | )

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- Definition Tabular 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. + References Reference 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=112275985&loc=d3e4179-114921 Reference 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=108410482&loc=d3e2439-114920 Reference 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=108410482&loc=d3e1928-114920 Reference 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=108410482&loc=d3e2410-114920 Reference 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=108410482&loc=d3e2417-114920 + Details
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