Fair Value Hierarchy of Financial Assets Measured at Fair Value on Recurring Basis (Detail) (Fair Value, Measurements, Recurring, USD $) In Thousands, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2013		Dec. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents (money market accounts and commercial paper investments)	$ 81,019		$ 23,598
Hedges of interest rate risk	(248)	[1]	(90)	[1]
Fair Value Assets And Liabilities Measured On Recurring Basis Net, Total	80,771		23,508
Cash equivalents (money market accounts and commercial paper investments)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Valuation Technique (Approach)	Market		Market
Hedges of interest rate risk
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Valuation Technique (Approach)	Market	[1]	Market	[1]
Fair Value, Inputs, Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents (money market accounts and commercial paper investments)	81,019		23,598
Fair Value Assets And Liabilities Measured On Recurring Basis Net, Total	81,019		23,598
Fair Value, Inputs, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Hedges of interest rate risk	(248)	[1]	(90)	[1]
Fair Value Assets And Liabilities Measured On Recurring Basis Net, Total	$ (248)		$ (90)

[1] ICG’s respective counterparties under the arrangements provide quarterly statements of market values of those instruments based on significant inputs that are observable or can be derived principally from, or corroborated by, observable market data for substantially the full term of the asset or liability