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Employee benefit plans - Schedule of Pension Plan Weighted-Average Asset Allocations (Detail)
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations 100.00% 100.00%
Equity Securities [Member]
    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations 59.90% 56.20%
Corporate Debt Securities [Member]
    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations 16.80% 18.90%
U.S. Government Agencies [Member]
    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations 20.80% 21.20%
Cash and Cash Equivalents [Member]
    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations 2.50% 3.70%