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Investments - Other Investments - Additional Information (Detail)
Dec. 31, 2017
USD ($)
Investment
Venture
Dec. 31, 2016
USD ($)
Schedule of Equity Method Investments [Line Items]    
Assets $ 79,100,000 $ 93,700,000
Non controlling interest certified low-income housing developments | Investment 8  
Other Investments [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets $ 3,800,000 $ 3,700,000
Other Investments [Member] | Four Venture Capital Funds [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets $ 3,800,000  
Non controlling interest in venture capital funds number | Venture 4  
Other Investments [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets $ 15,000,000  
Liabilities 0  
Other Investments [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] | Eight Certified Low Income Housing Developments [Member]    
Schedule of Equity Method Investments [Line Items]    
Carrying value of investments $ 0