Investments - Fair Values of Fixed Maturity Investments, Short-Term Investments and Equities Classified as Trading (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, debt	$ 5,109,361
Equities, trading, at fair value	117,293	$ 115,941
Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, debt	5,109,361	5,078,144
Trading securities	5,226,654	5,194,085
U.S. government and agency | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, debt	771,489	750,957
Non-U.S. government | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, debt	309,794	359,002
Corporate | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, debt	2,612,211	2,631,682
Municipal | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, debt	8,691	22,247
Residential mortgage-backed | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, debt	474,820	391,247
Commercial mortgage-backed | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, debt	281,052	284,575
Asset-backed | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, debt	651,304	638,434
Equity Securities | Fair Value | US
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equities, trading, at fair value	109,903	108,793
Equity Securities | Fair Value | International
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equities, trading, at fair value	7,390	7,148
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, debt	447,000	359,400
Senior Secured Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, debt	$ 89,900	$ 94,400