Fair Value Measurement of Assets and Liabilities (Detail) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts Receivable, Net, Noncurrent		$ 1	$ 2
Assets:
Cash and cash equivalents of continuing operations at end of period		84	94
Estimate of Fair Value Measurement [Member]
Assets:
Total assets:		199	277
Liabilities:
Total liabilities:		21	14
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Cash and cash equivalents of continuing operations at end of period		84	94
Restricted funds held in trust		110	160
Estimate of Fair Value Measurement [Member] | Bank deposits and certificates of deposit [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Bank deposits and certificates of deposit		79	89
Restricted funds held in trust		12	9
Estimate of Fair Value Measurement [Member] | Money market funds [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Money Market Funds, at Carrying Value		5	5
Restricted funds held in trust		36	66
Estimate of Fair Value Measurement [Member] | U.S. Treasury/Agency obligations [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Restricted funds held in trust	[1]	14	18
Estimate of Fair Value Measurement [Member] | State and municipal obligations [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Restricted funds held in trust		46	59
Estimate of Fair Value Measurement [Member] | Commercial paper/Guaranteed investment contracts/Repurchase agreements [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Restricted funds held in trust		2	8
Estimate of Fair Value Measurement [Member] | Mutual And Bond Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investments	[2]	2	2
Estimate of Fair Value Measurement [Member] | Energy Hedges [Member] | Fair Value, Inputs, Level 2 [Member]
Liabilities:
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[3]	1	0
Estimate of Fair Value Measurement [Member] | Interest Rate Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Liabilities:
Derivative Liability	[3],[4]		14
Energy Hedges [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Derivative Asset		2	21
Energy Hedges [Member] | Estimate of Fair Value Measurement [Member] | Energy Hedges [Member]
Assets:
Derivative Asset	[5]	3
Energy Hedges [Member] | Estimate of Fair Value Measurement [Member] | Energy Hedges [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Derivative Asset	[5]		$ 21
Interest Rate Swap [Member] | Interest Rate Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Liabilities:
Derivative Liability	[3],[4]	$ 20

[1] (1)The U.S. Treasury/agency obligations in restricted funds held in trust are primarily comprised of Federal Home Loan Mortgage Corporation securities at fair value.

[2] (2)Included in other noncurrent assets in the consolidated balance sheets.

[3] Included in accrued expenses and other current liabilities in the consolidated balance sheets.

[4] Included in other noncurrent liabilities in the consolidated balance sheets.

[5] (3)Included in prepaid expenses and other current assets in the consolidated balance sheets.