Investments - Estimated Fair Values of Investments in Fixed Maturity Investments, Short-Term Investments and Equities, Trading Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	$ 5,078,144
Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	5,194,085	$ 4,112,937
Municipal Bonds | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	22,247	25,607
Total fixed maturity and short-term investments | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	5,078,144	3,962,807
U.S. Government and Agency | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	750,957	744,660
Non-U.S. Government | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	359,002	368,945
Corporate | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	2,631,682	1,986,873
Residential Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	391,247
Residential Mortgage-Backed | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	391,247	308,621
Commercial Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	284,575
Commercial Mortgage-Backed | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	284,575	139,907
Asset-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	638,434
Asset-Backed | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	638,434	388,194
Equities - U.S. | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	108,793	106,895
Equities - International | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	$ 7,148	$ 43,235