Financial Instruments - Additional Information (Detail) (USD $)	9 Months Ended
Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Maturities of cash equivalents, in months	Three
Long-term debt, carrying values	$ 1,113,361,000	$ 1,047,859,000	$ 1,098,788,000
Long-term debt, fair values	$ 1,152,906,000	$ 1,105,650,000	$ 1,156,820,000

- Definition

An element designated to encapsulate any additional information related to cash and cash equivalents not otherwise addressed by the existing taxonomy. Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Entity may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

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