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PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS - Reconciliation of Pension Plan Assets Measured at Level 3 Fair Value (Details) - Fair Value, Inputs, Level 3 - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets at beginning of year (estimated) $ 455 $ 546
Net realized gains (losses) (16) 131
Net change in unrealized gains (losses) 110 (122)
Purchases 14 21
Sales 1 18
Settlements (13) (128)
Transfers into Level 3 25 25
Transfers out of Level 3 (17) 0
Fair value of plan assets at end of year (estimated) 557 455
Hedge funds and related investments    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets at beginning of year (estimated) 4 3
Net realized gains (losses) (1) (1)
Net change in unrealized gains (losses) 2 2
Purchases 0 0
Sales 1 0
Settlements 0 0
Transfers into Level 3 6 0
Transfers out of Level 3 0 0
Fair value of plan assets at end of year (estimated) 10 4
Private equity and related investments    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets at beginning of year (estimated) 75 52
Net realized gains (losses) (30) (2)
Net change in unrealized gains (losses) 41 (3)
Purchases 14 21
Sales 0 18
Settlements 0 0
Transfers into Level 3 19 25
Transfers out of Level 3 (17) 0
Fair value of plan assets at end of year (estimated) 102 75
Derivative instruments, net    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets at beginning of year (estimated) 376 491
Net realized gains (losses) 15 134
Net change in unrealized gains (losses) 67 (121)
Purchases 0 0
Sales 0 0
Settlements (13) (128)
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Fair value of plan assets at end of year (estimated) $ 445 $ 376