Compensation Plans (Liability For Share-Based Payment Transactions) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation Plans [Abstract]
Liability for Unvested Cash-Settled Share-Based Payment Transactions	$ 274	$ 171	$ 47
Liability for Vested Cash-Settled Share-Based Payment Transactions	53	37	4
Liability for Cash-Settled Share-Based Payment Transactions	$ 327	$ 208	$ 51

Aggregate carrying value as of the balance sheet date of the unvested portion of liabilities for stock option plans and other equity-based compensattion arrangements.

No definition available.

Name:	eca_LiabilityForUnvestedCashSettledShareBasedPaymentTransactions
Namespace Prefix:	eca_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant