v3.20.4
Other Comprehensive Income (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Other comprehensive income (loss) before reclassifications $ 647 $ 2,635 $ (801)
Amounts reclassified from accumulated OCI (1,787) (1,414) (1,309)
Other comprehensive income (loss) attributable to AT&T (1,140) 1,221 (2,110)
Accumulated Other Comprehensive Income      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Accumulated other comprehensive income, beginning balance 5,470 4,249 7,017
Other comprehensive income (loss) attributable to AT&T (1,140) 1,221 (2,110)
Accumulated other comprehensive income, ending balance 4,330 5,470 4,249
Accumulated Other Comprehensive Income | Amounts reclassified to retained earnings      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Accumulated other comprehensive income, beginning balance     (658)
Foreign Currency Translation Adjustment      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Accumulated other comprehensive income, beginning balance (3,056) (3,084) (2,054)
Other comprehensive income (loss) before reclassifications (870) 28 (1,030)
Amounts reclassified from accumulated OCI 0 0 0
Other comprehensive income (loss) attributable to AT&T (870) 28 (1,030)
Accumulated other comprehensive income, ending balance (3,926) (3,056) (3,084)
Foreign Currency Translation Adjustment | Amounts reclassified to retained earnings      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Accumulated other comprehensive income, beginning balance     0
Net Unrealized Gains (Losses) on Available-for-Sale Securities      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Accumulated other comprehensive income, beginning balance 48 (2) 660
Other comprehensive income (loss) before reclassifications 78 50 (4)
Amounts reclassified from accumulated OCI (15) 0 0
Other comprehensive income (loss) attributable to AT&T 63 50 (4)
Accumulated other comprehensive income, ending balance 111 48 (2)
Net Unrealized Gains (Losses) on Available-for-Sale Securities | Amounts reclassified to retained earnings      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Accumulated other comprehensive income, beginning balance     (658)
Net Unrealized Gains (Losses) on Cash Flow Hedges      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Accumulated other comprehensive income, beginning balance     1,402
Other comprehensive income (loss) before reclassifications     (597)
Amounts reclassified from accumulated OCI     13
Other comprehensive income (loss) attributable to AT&T     (584)
Net Unrealized Gains (Losses) on Cash Flow Hedges | Amounts reclassified to retained earnings      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Accumulated other comprehensive income, beginning balance     0
Net Unrealized Gains (Losses) on Cash Flow Hedges      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Accumulated other comprehensive income, beginning balance (37) 818  
Other comprehensive income (loss) before reclassifications (811) (900)  
Amounts reclassified from accumulated OCI 69 45  
Other comprehensive income (loss) attributable to AT&T (742) (855)  
Accumulated other comprehensive income, ending balance (779) (37) 818
Defined Benefit Postretirement Plans      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Accumulated other comprehensive income, beginning balance 8,515 6,517 7,009
Other comprehensive income (loss) before reclassifications 2,250 3,457 830
Amounts reclassified from accumulated OCI (1,841) (1,459) (1,322)
Other comprehensive income (loss) attributable to AT&T 409 1,998 (492)
Accumulated other comprehensive income, ending balance $ 8,924 $ 8,515 6,517
Defined Benefit Postretirement Plans | Amounts reclassified to retained earnings      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Accumulated other comprehensive income, beginning balance     $ 0