Estimated Fair Values of Other Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion	$ 1,394,077	$ 1,401,783	$ 1,411,122	$ 1,419,084	$ 1,427,319
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion	$ 1,601,418	$ 1,674,413	$ 1,834,869		$ 1,786,232