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Debt - Additional Information (Detail) | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2016 USD ($) | Aug. 31, 2016 USD ($) | May 31, 2016 USD ($) | Mar. 31, 2016 CAD | Mar. 31, 2016 USD ($) | Mar. 31, 2016 CAD | Mar. 31, 2015 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2016 CAD | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2016 CAD | Dec. 31, 2015 CAD
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Current portion of long-term debt |  |  |  |  |  |  |  | $ 417,000,000 |  | $ 417,000,000 | $ 253,000,000 |  |
Outstanding borrowings under credit facility |  |  |  |  |  |  |  | 230,000,000 |  | 230,000,000 |  |  |
Debt maturing within twelve months |  |  |  |  |  |  |  | 187,000,000 |  | 187,000,000 |  |  |
Tax-exempt bonds subject to re-pricing within next 12 months |  |  |  |  |  |  |  | 473,000,000 |  | 473,000,000 |  |  |
Loss on early extinguishment of debt |  |  |  |  |  |  |  |  |  | (4,000,000) | (555,000,000) |  |
Debt and capital lease principal payments in 2017 |  |  |  |  |  |  |  | 610,000,000 |  | 610,000,000 |  |  |
Debt and capital lease principal payments in 2018 |  |  |  |  |  |  |  | 783,000,000 |  | 783,000,000 |  |  |
Debt and capital lease principal payments in 2019 |  |  |  |  |  |  |  | 432,000,000 |  | 432,000,000 |  |  |
Debt and capital lease principal payments in 2020 |  |  |  |  |  |  |  | 746,000,000 |  | 746,000,000 |  |  |
Debt and capital lease principal payments in 2021 |  |  |  |  |  |  |  | 540,000,000 |  | 540,000,000 |  |  |
Tax-exempt Bonds [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt maturing within twelve months |  |  |  |  |  |  |  | 126,000,000 |  | 126,000,000 |  |  |
Repayment of tax exempt bonds |  |  | $ 143,000,000 |  |  |  |  |  |  |  |  |  |
Proceeds from tax-exempt bonds |  |  | 143,000,000 |  |  |  |  |  |  |  |  |  |
Loss on early extinguishment of debt |  |  |  |  |  |  |  |  |  | (3,000,000) |  |  |
Tax-exempt Bonds [Member] | Variable Rate [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable-rate tax-exempt bonds |  |  |  |  |  |  |  | $ 491,000,000 |  | $ 491,000,000 |  |  |
2.4% Senior Notes Due May 2023 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  | $ 500,000,000 |  |  |  |  |  |  |  |  |  |
Interest rate |  |  | 2.40% |  |  |  |  |  |  |  |  |  |
Maturity period |  |  | 2023-05 |  |  |  |  |  |  |  |  |  |
2.6% Senior Notes Mature in September 2016 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate | 2.60% |  |  |  |  |  |  |  |  |  |  |  |
Maturity period | 2016-09 |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from issuance of senior notes | $ 496,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Net borrowings (repayments) of senior notes | $ (500,000,000) |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on early extinguishment of debt |  |  |  |  |  |  | $ (550,000,000) |  |  |  | (552,000,000) |  |
Early extinguishment of high-coupon senior notes |  |  |  |  |  |  | $ 2,000,000,000 |  |  |  | $ 2,000,000,000 |  |
2016 Canadian Credit Agreement [Member] | Intercompany Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Repayment of debt |  |  |  | CAD 370,000,000 | $ 280,000,000 | CAD 370,000,000 |  |  |  |  |  |  |
Canadian Term Loan and Credit Facility [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding borrowings under credit facility | CAD |  |  |  |  |  |  |  |  |  |  |  | CAD 0 | CAD 0
Credit Facility, aggregate capacity | CAD |  |  |  | CAD 50,000,000 |  | 50,000,000 |  |  |  |  |  |  |
Letters of credit outstanding revolving credit facility | CAD |  |  |  |  |  |  |  |  |  |  |  | CAD 0 | CAD 0
Maturity date of credit facility |  |  |  | Mar. 31, 2019 |  |  |  |  |  |  |  |  |
Minimum [Member] | Tax-exempt Bonds [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  | 0.70% |  | 0.70% |  | 0.70% |
Minimum [Member] | Canadian Term Loan and Credit Facility [Member] | London Interbank Offered Rate [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Spread rate |  |  |  |  |  |  |  |  |  | 0.875% |  |  |
Maximum [Member] | Tax-exempt Bonds [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  | 5.70% |  | 5.70% |  | 5.70% |
Maximum [Member] | Canadian Term Loan and Credit Facility [Member] | London Interbank Offered Rate [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Spread rate |  |  |  |  |  |  |  |  |  | 1.50% |  |  |
Cash [Member] | Tax-exempt Bonds [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Repayment of tax exempt bonds |  |  |  |  |  |  |  |  |  | $ 146,000,000 |  |  |
Revolving Credit Facility [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding borrowings under credit facility |  |  |  |  |  |  |  | $ 426,000,000 |  | 426,000,000 |  |  |
Credit Facility, aggregate capacity |  |  |  |  |  |  |  | 2,250,000,000 |  | 2,250,000,000 |  |  |
Outstanding borrowings under credit facility classified as long-term |  |  |  |  |  |  |  | 196,000,000 |  | 196,000,000 |  |  |
Letters of credit outstanding revolving credit facility |  |  |  |  |  |  |  | 789,000,000 |  | 789,000,000 |  |  |
Credit Facility, remaining capacity |  |  |  |  |  |  |  | 1,035,000,000 |  | $ 1,035,000,000 |  |  |
Maturity date of credit facility |  |  |  |  |  |  |  |  |  | Jul. 30, 2020 |  |  |
Net borrowings (repayments) |  |  |  |  |  |  |  |  |  | $ 406,000,000 |  |  |
Domestic Line of Credit [Member] | Minimum [Member] | London Interbank Offered Rate [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Spread rate |  |  |  |  |  |  |  |  |  | 0.805% |  |  |
Domestic Line of Credit [Member] | Maximum [Member] | London Interbank Offered Rate [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Spread rate |  |  |  |  |  |  |  |  |  | 1.30% |  |  |
Committed and Uncommitted [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Letters of credit outstanding revolving credit facility |  |  |  |  |  |  |  | 492,000,000 |  | $ 492,000,000 |  |  |
Canadian Term Loan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Credit Facility, aggregate capacity | CAD |  |  |  | CAD 500,000,000 |  | 500,000,000 |  |  |  |  |  |  |
Credit Facility, remaining capacity | CAD |  |  |  | 90,000,000 |  | 90,000,000 |  |  |  |  |  |  |
Net borrowings (repayments) |  |  |  |  | (20,000,000) | (27,000,000) |  | (105,000,000) | CAD (139,000,000) |  |  |  |
Canadian Term Loan [Member] | 2016 Canadian Credit Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net borrowings (repayments) |  |  |  |  | $ 347,000,000 | 460,000,000 |  |  |  |  |  |  |
Non Revolving Term Credit Facility [Member] | 2016 Canadian Credit Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Credit Facility, aggregate capacity | CAD |  |  |  | CAD 460,000,000 |  | CAD 460,000,000 |  |  |  |  |  |  |
Commercial Paper [Member] | Revolving Credit Facility [Member] | Commercial Paper Program [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Credit Facility, Borrowing Capacity |  | $ 1,500,000,000 |  |  |  |  |  |  |  |  |  |  |
Commercial paper, borrowings |  |  |  |  |  |  |  | $ 0 |  | $ 0 |  |  |
Commercial Paper [Member] | Revolving Credit Facility [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt term |  | 397 days |  |  |  |  |  |  |  |  |  |  |

X
- Definition Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(19)(a)(3)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(13)(2)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Amount of short-term debt and current maturity of long-term debt and capital lease obligations due within one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19,20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition The cash outflow for the extinguishment of long-term borrowing, with the highest claim on the assets of the entity in case of bankruptcy or liquidation, before its maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3291-108585 + Details
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- Definition Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 50 -Section 40 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=49170532&loc=d3e12317-112629 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 50 -Section 40 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=49170532&loc=d3e12355-112629 + Details
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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Date the credit facility terminates, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Principal amount of long-term debt and capital lease obligation maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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- Definition Principal amount of long-term debt and capital lease obligation maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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- Definition Principal amount of long-term debt and capital lease obligation maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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- Definition Principal amount of long-term debt and capital lease obligation maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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- Definition Principal amount of long-term debt and capital lease obligation maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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- Definition The portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a rate subject to change from time to time. + References No definition available. + Details
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