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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
Jun. 30, 2015
Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 459,614 $ 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 $ 43,819 $ 39,657
Percentage of total Fair Value 9.50% 9.80%
Comparable Ratings, AA Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 233,077 $ 220,693
Percentage of total Fair Value 50.80% 54.80%
Comparable Ratings, A Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 117,665 $ 83,734
Percentage of total Fair Value 25.60% 20.70%
Comparable Ratings, BBB Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 47,328 $ 47,003
Percentage of total Fair Value 10.30% 11.60%
Comparable Ratings, BB+ and Below Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 4,719 $ 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 $ 13,006 $ 9,451
Percentage of total Fair Value 2.80% 2.30%