Financial Instruments - Financial Instruments not Carried at Fair Value but which Require Fair Value Disclosure (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Cash, cash equivalents and restricted cash, Carrying amount	$ 433.4	$ 323.7	$ 318.0	$ 52.3
Fixed rate Senior Notes, Carrying amount	463.0	471.1
Common stockholders' equity, Carrying amount	873.4	754.3
Cash, cash equivalents and restricted cash, Fair value	433.4	323.7
Fixed rate Senior Notes, Fair value	455.1	495.1
Common stockholders' equity, Fair value	$ 1,883.5	$ 2,986.8

Cash, cash equivalents and restricted cash fair value disclosure.

No definition available.

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- Definition

Amount of cash and cash equivalents, and cash and cash equivalents restricted to withdrawal or usage. Excludes amount for disposal group and discontinued operations. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

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- Definition

Fair value of the entity's equity.

+ References

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Fair value portion of notes payable.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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- Definition

Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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