Investments - Amortized Cost and Estimated Fair Values of Company's Fixed Maturity Classified as Trading (Detail) $ in Thousands	Dec. 31, 2015 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost, One year or less	$ 633,011
Amortized Cost, More than one year through two years	957,838
Amortized Cost, More than two years through five years	1,538,344
Amortized Cost, More than five years through ten years	510,271
Amortized Cost, More than ten years	169,589
Total amortized cost	5,148,832
Fair Value, One year or less	620,887
Fair Value, More than one year through two years	948,734
Fair Value, More than two years through five years	1,525,009
Fair Value, More than five years through ten years	503,108
Fair Value, More than ten years	166,150
Total fair value	$ 5,078,144
Percentage of Total Fair Value, One year or less	12.20%
Percentage of Total Fair Value, More than one year through two years	18.70%
Percentage of Total Fair Value, More than two years through five years	30.00%
Percentage of Total Fair Value, More than five years through ten years	9.90%
Percentage of Total Fair Value, More than ten years	3.30%
Percentage of Total Fair Value	100.00%
Residential Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total amortized cost	$ 394,010
Total fair value	$ 391,247
Percentage of Total Fair Value	7.70%
Commercial Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total amortized cost	$ 286,373
Total fair value	$ 284,575
Percentage of Total Fair Value	5.60%
Asset-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total amortized cost	$ 659,396
Total fair value	$ 638,434
Percentage of Total Fair Value	12.60%