Supplemental Condensed Consolidating Financial Information and Other Collateral-Related Information - Additional Information (Detail) - USD ($)	Dec. 31, 2017	Dec. 06, 2012
Supplemental Condensed Consolidating Financial Information [Line Items]
Ownership percentage held by parent	100.00%
Senior Unsecured Notes Due 2021 [Member]
Supplemental Condensed Consolidating Financial Information [Line Items]
Debt instrument, principal amount	$ 1,000,000,000	$ 1,000,000,000
Debt instrument, stated interest		6.25%

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant