Investments - Amortized Cost and Estimated Fair Values of Company's Fixed Maturity Classified as Trading (Detail) $ in Thousands	Mar. 31, 2016 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost, One year or less	$ 752,003
Amortized Cost, More than one year through two years	941,118
Amortized Cost, More than two years through five years	1,392,710
Amortized Cost, More than five years through ten years	563,722
Amortized Cost, More than ten years	196,207
Total amortized cost	5,221,590
Fair Value, One year or less	743,560
Fair Value, More than one year through two years	940,001
Fair Value, More than two years through five years	1,403,136
Fair Value, More than five years through ten years	568,004
Fair Value, More than ten years	198,529
Total fair value	$ 5,200,928
Percentage of Total Fair Value, One year or less	14.30%
Percentage of Total Fair Value, More than one year through two years	18.10%
Percentage of Total Fair Value, More than two years through five years	27.00%
Percentage of Total Fair Value, More than five years through ten years	10.90%
Percentage of Total Fair Value, More than ten years	3.80%
Percentage of Total Fair Value	100.00%
Residential Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total amortized cost	$ 450,055
Total fair value	$ 450,720
Percentage of Total Fair Value	8.70%
Commercial Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total amortized cost	$ 278,868
Total fair value	$ 276,134
Percentage of Total Fair Value	5.30%
Asset-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total amortized cost	$ 646,907
Total fair value	$ 620,844
Percentage of Total Fair Value	11.90%