Fair Value Measurements - Financial Assets and Liabilities Accounted at Fair Value (Detail) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Cash equivalents - money - market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents - money-market funds	$ 760	$ 2,449
Registered investment companies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring basis investments	4,883	5,057
Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring basis investments		9
Long term debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long term debt	1,575,950	1,181,788
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Cash equivalents - money - market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents - money-market funds
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Registered investment companies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring basis investments	4,883	5,057
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring basis investments		9
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Long term debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long term debt
Significant Other Observable Inputs (Level 2) [Member] | Cash equivalents - money - market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents - money-market funds	760	2,449
Significant Other Observable Inputs (Level 2) [Member] | Registered investment companies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring basis investments
Significant Other Observable Inputs (Level 2) [Member] | Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recurring basis investments
Significant Other Observable Inputs (Level 2) [Member] | Long term debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long term debt	$ 1,575,950	$ 1,181,788