Financial Risk Management Activities - Gross Notional Amounts of Derivative Contracts Outstanding Excluding Commodity Contracts (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Foreign Exchange
Derivative [Line Items]
Outstanding gross notional amount	$ 31,000,000	$ 52,000,000
Interest Rate Swaps
Derivative [Line Items]
Outstanding gross notional amount	$ 100,000,000	$ 450,000,000

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant