Financial Instruments and Fair Value Measurements - Summary of Activity in Interest Rate Swaps (Detail) - Interest Rate Swaps [Member] |
12 Months Ended | |||
|---|---|---|---|---|
|
Dec. 31, 2017
USD ($)
|
Dec. 31, 2017
CAD
|
Dec. 31, 2016
USD ($)
|
Dec. 31, 2015
USD ($)
|
|
| Derivative [Line Items] | ||||
| Notional amounts at January 1 | $ 271,000,000 | $ 1,196,000,000 | $ 398,000,000 | |
| New contracts | 0 | 1,158,000,000 | ||
| Matured, expired or settled contracts | (925,000,000) | (360,000,000) | ||
| Notional amounts at December 31 | $ 271,200,000 | CAD 371,900,000 | $ 271,000,000 | $ 1,196,000,000 |