Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect adjustment from changes in accounting standards	$ (91)
Beginning Balance	24,566	$ 23,361	$ 22,810
Balance, as adjusted	24,475
Other comprehensive income (loss)	(892)	218	153
Purchase of Boardwalk Pipeline common units	(1,718)		(9)
Ending Balance	21,386	24,566	23,361
OTTI Gains (Losses) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	22	27	24
Balance, as adjusted	26
Other comprehensive income (loss) before reclassifications, after tax	(7)	(3)	9
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	(7)	(2)	(6)
Other comprehensive income (loss)	(14)	(5)	3
Amounts attributable to noncontrolling interests	2
Ending Balance	14	22	27
Unrealized Gains (Losses) on Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	673	576	347
Balance, as adjusted	771
Other comprehensive income (loss) before reclassifications, after tax	(801)	190	283
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	3	(82)	(26)
Other comprehensive income (loss)	(798)	108	257
Amounts attributable to noncontrolling interests	84	(11)	(28)
Ending Balance	57	673	576
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(2)	(3)
Other comprehensive income (loss) before reclassifications, after tax	4	1
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	2	2	2
Other comprehensive income (loss)	6	3	2
Amounts attributable to noncontrolling interests		(1)	(1)
Purchase of Boardwalk Pipeline common units	(1)
Ending Balance	5		(2)
Pension Liability [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(633)	(646)	(649)
Balance, as adjusted	(763)
Other comprehensive income (loss) before reclassifications, after tax	(34)	(18)	(22)
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	32	30	27
Other comprehensive income (loss)	(2)	12	5
Amounts attributable to noncontrolling interests		1	(2)
Purchase of Boardwalk Pipeline common units	(28)
Ending Balance	(793)	(633)	(646)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(88)	(178)	(76)
Balance, as adjusted	(88)
Other comprehensive income (loss) before reclassifications, after tax	(84)	100	(114)
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	0
Other comprehensive income (loss)	(84)	100	(114)
Amounts attributable to noncontrolling interests	9	(10)	12
Ending Balance	(163)	(88)	(178)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(26)	(223)	(357)
Balance, as adjusted	(54)
Other comprehensive income (loss) before reclassifications, after tax	(922)	270	156
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	30	(52)	(3)
Other comprehensive income (loss)	(892)	218	153
Amounts attributable to noncontrolling interests	95	(21)	(19)
Purchase of Boardwalk Pipeline common units	(29)
Ending Balance	(880)	$ (26)	$ (223)
Accounting Standards Update 2016-01 [Member] | Unrealized Gains (Losses) on Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect adjustment from changes in accounting standards	(25)
Accounting Standards Update 2016-01 [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect adjustment from changes in accounting standards	(25)
Accounting Standards Update 2018-02 [Member] | OTTI Gains (Losses) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect adjustment from changes in accounting standards	4
Accounting Standards Update 2018-02 [Member] | Unrealized Gains (Losses) on Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect adjustment from changes in accounting standards	123
Accounting Standards Update 2018-02 [Member] | Pension Liability [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect adjustment from changes in accounting standards	(130)
Accounting Standards Update 2018-02 [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect adjustment from changes in accounting standards	$ (3)