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Investments - Schedule of Credit Quality of Investments With Contractual Maturities (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2014
Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 352,983 $ 289,418
Percentage of total Fair Value 100.00% 100.00%
Standard & Poor's, AAA Rating [Member]
    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value 28,505 82,889
Percentage of total Fair Value 8.10% 28.60%
Standard & Poor's, AA Rating [Member]
    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value 192,903 120,976
Percentage of total Fair Value 54.70% 41.80%
Standard & Poor's, A Rating [Member]
    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value 76,030 46,689
Percentage of total Fair Value 21.50% 16.10%
Standard & Poor's, BBB Rating [Member]
    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value 42,091 38,114
Percentage of total Fair Value 11.90% 13.20%
Standard & Poor's, BB- Rating [Member]
    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value 2,714  
Percentage of total Fair Value 0.80%  
Standard & Poor's, Not Rated [Member]
    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 10,740 $ 750
Percentage of total Fair Value 3.00% 0.30%