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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
Sep. 30, 2015
Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 492,792 $ 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 $ 87,860 $ 39,657
Percentage of total Fair Value 17.80% 9.80%
Comparable Ratings, AA Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 193,220 $ 220,693
Percentage of total Fair Value 39.20% 54.80%
Comparable Ratings, A Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 140,092 $ 83,734
Percentage of total Fair Value 28.40% 20.70%
Comparable Ratings, BBB Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 47,219 $ 47,003
Percentage of total Fair Value 9.60% 11.60%
Comparable Ratings, BB+ and Below Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 3,768 $ 3,401
Percentage of total Fair Value 0.80% 0.80%
Comparable Ratings, No Rating Available [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 20,633 $ 9,451
Percentage of total Fair Value 4.20% 2.30%