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Financial instruments - Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Investments [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Beginning Balance $ 12,954 $ 12,482
Total Realized and Unrealized Gains (Losses) 329 515
Purchases and Issuances   124
Sales and Settlements (504)  
Transfers In (Out)   11
Ending Balance 12,779 13,132
Other [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Beginning Balance 100 50
Purchases and Issuances   (50)
Sales and Settlements      
Transfers In (Out)      
Ending Balance 100  
Derivative contracts [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Beginning Balance 2,647 2,347
Total Realized and Unrealized Gains (Losses) 553  
Purchases and Issuances   1,560
Sales and Settlements      
Transfers In (Out)      
Ending Balance 2,094 3,907
Mortgage and other asset-backed securities [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Beginning Balance 40 16
Total Realized and Unrealized Gains (Losses) 7 17
Purchases and Issuances 23 80
Sales and Settlements (18) (15)
Ending Balance 52 98
Sales and Settlements      
Transfers In (Out)      
Municipal obligations [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Beginning Balance 9,394 3,562
Total Realized and Unrealized Gains (Losses) (141) (1,078)
Purchases and Issuances 1,200 9,305
Sales and Settlements (150)  
Ending Balance 10,303 11,789
Other [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Beginning Balance 62,963 65,001
Total Realized and Unrealized Gains (Losses) (549) (1,499)
Purchases and Issuances 3,050 10,300
Sales and Settlements (2,975) (6,971)
Ending Balance $ 62,489 $ 66,831