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Investments - Schedule of Credit Quality of Investment Securities With Contractual Maturities or The Issuer of Such Securities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 441,104 $ 403,939
Percentage of Total Fair Value 100.00% 100.00%
Comparable Ratings, AAA Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 103,097 $ 39,657
Percentage of Total Fair Value 23.40% 9.80%
Comparable Ratings, AA Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 189,600 $ 220,693
Percentage of Total Fair Value 43.00% 54.80%
Comparable Ratings, A Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 83,850 $ 83,734
Percentage of Total Fair Value 19.00% 20.70%
Comparable Ratings, BBB Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 41,408 $ 47,003
Percentage of Total Fair Value 9.40% 11.60%
Comparable Ratings, BB and Below Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 4,261 $ 3,401
Percentage of Total Fair Value 1.00% 0.80%
Comparable Ratings, No Rating Available [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 18,888 $ 9,451
Percentage of Total Fair Value 4.20% 2.30%