Investments - Estimated Fair Values of Investments in Fixed Maturity Investments, Short-Term Investments and Equities, Trading Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	$ 5,200,928
Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	5,319,188	$ 5,194,085
Municipal Bonds | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	11,416	22,247
Total fixed maturity and short-term investments | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	5,200,928	5,078,144
U.S. Government and Agency | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	806,426	750,957
Non-U.S. Government | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	349,935	359,002
Corporate | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	2,685,453	2,631,682
Residential Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	450,720
Residential Mortgage-Backed | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	450,720	391,247
Commercial Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	276,134
Commercial Mortgage-Backed | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	276,134	284,575
Asset-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	620,844
Asset-Backed | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	620,844	638,434
Equities - U.S. | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	110,987	108,793
Equities - International | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	$ 7,273	$ 7,148