Investments - Schedule of Amortized Cost and Fair Value of Investment Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Investments Debt And Equity Securities [Abstract]
Due in one year or less, Cost or Amortized Cost	$ 70,683
Due after one year through five years, Cost or Amortized Cost	177,559
Due after five years through ten years, Cost or Amortized Cost	165,557
Due after ten years, Cost or Amortized Cost	95,385
Cost or Adjusted / Amortized Cost	$ 509,184	$ 370,967
Due in one year or less, Percentage of Total	14.00%
Due after one year through five years, Percentage of Total	35.00%
Due after five years through ten years, Percentage of Total	32.00%
Due after ten years, Percentage of Total	19.00%
Total, Percentage	100.00%
Due in one year or less, Fair Value	$ 70,696
Due after one year through five years, Fair Value	178,915
Due after five years through ten years, Fair Value	169,966
Due after ten years, Fair Value	97,996
Total, Fair Value	$ 517,573	$ 371,783
Due in one year or less, Percentage of Total	14.00%
Due after one year through five years, Percentage of Total	34.00%
Due after five years through ten years, Percentage of Total	33.00%
Due after ten years, Percentage of Total	19.00%
Total, Percentage	100.00%