Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 21,386	$ 24,566	$ 23,361
Cumulative effect adjustment from changes in accounting standards		(91)
Balance, as adjusted		24,475
Other comprehensive income (loss)	911	(892)	218
Purchase of Boardwalk Pipelines common units		(1,718)
Ending Balance	21,930	21,386	24,566
Net Unrealized Gains (Losses) on Investments with OTTI Losses [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	14	22	27
Balance, as adjusted		26
Other comprehensive income (loss) before reclassifications, after tax	(13)	(7)	(3)
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	12	(7)	(2)
Other comprehensive income (loss)	(1)	(14)	(5)
Amounts attributable to noncontrolling interests		2
Ending Balance	13	14	22
Net Other Unrealized Gains (Losses) on Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	57	673	576
Balance, as adjusted		771
Other comprehensive income (loss) before reclassifications, after tax	957	(801)	190
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	(8)	3	(82)
Other comprehensive income (loss)	949	(798)	108
Amounts attributable to noncontrolling interests	(101)	84	(11)
Ending Balance	905	57	673
Unrealized Gains (Losses) on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	5		(2)
Other comprehensive income (loss) before reclassifications, after tax	(11)	4	1
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax		2	2
Other comprehensive income (loss)	(11)	6	3
Amounts attributable to noncontrolling interests			(1)
Purchase of Boardwalk Pipelines common units		(1)
Ending Balance	(6)	5
Pension and Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(793)	(633)	(646)
Balance, as adjusted		(763)
Other comprehensive income (loss) before reclassifications, after tax	(102)	(34)	(18)
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	34	32	30
Other comprehensive income (loss)	(68)	(2)	12
Amounts attributable to noncontrolling interests	6		1
Purchase of Boardwalk Pipelines common units		(28)
Ending Balance	(855)	(793)	(633)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(163)	(88)	(178)
Balance, as adjusted		(88)
Other comprehensive income (loss) before reclassifications, after tax	42	(84)	100
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax
Other comprehensive income (loss)	42	(84)	100
Amounts attributable to noncontrolling interests	(4)	9	(10)
Ending Balance	(125)	(163)	(88)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(880)	(26)	(223)
Balance, as adjusted		(54)
Other comprehensive income (loss) before reclassifications, after tax	873	(922)	270
Reclassification of (gains) losses from accumulated other comprehensive loss, after tax	38	30	(52)
Other comprehensive income (loss)	911	(892)	218
Amounts attributable to noncontrolling interests	(99)	95	(21)
Purchase of Boardwalk Pipelines common units		(29)
Ending Balance	$ (68)	(880)	$ (26)
Accounting Standards Update 2016-01 [Member] | Net Unrealized Gains (Losses) on Investments with OTTI Losses [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect adjustment from changes in accounting standards		4
Accounting Standards Update 2016-01 [Member] | Net Other Unrealized Gains (Losses) on Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect adjustment from changes in accounting standards		98
Accounting Standards Update 2016-01 [Member] | Pension and Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect adjustment from changes in accounting standards		(130)
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		$ (28)