Investments - Contractual Maturities of Fixed Maturity and Short-term Investments Classified as Trading (Detail) $ in Thousands	Jun. 30, 2016 USD ($)
Amortized Cost
One year or less	$ 732,723
More than one year through two years	927,837
More than two years through five years	1,266,491
More than five years through ten years	551,552
More than ten years	197,290
Trading securities, amortized cost	5,100,599
Fair Value
One year or less	723,589
More than one year through two years	928,798
More than two years through five years	1,279,337
More than five years through ten years	563,660
More than ten years	206,801
Trading securities, debt	$ 5,109,361
% of Total Fair Value
One year or less	14.20%
More than one year through two years	18.20%
More than two years through five years	25.00%
More than five years through ten years	11.00%
More than ten years	4.00%
Trading securities, percentage of fair value	100.00%
Residential mortgage-backed
Amortized Cost
Securities without single maturity date	$ 473,782
Fair Value
Securities without single maturity date	$ 474,820
% of Total Fair Value
Securities without single maturity date	9.30%
Commercial mortgage-backed
Amortized Cost
Securities without single maturity date	$ 280,949
Fair Value
Securities without single maturity date	$ 281,052
% of Total Fair Value
Securities without single maturity date	5.50%
Asset-backed
Amortized Cost
Securities without single maturity date	$ 669,975
Fair Value
Securities without single maturity date	$ 651,304
% of Total Fair Value
Securities without single maturity date	12.80%