Investments - Estimated Fair Values of Investments in Fixed Maturity Investments, Short-Term Investments and Equities, Trading Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated fair value of trading securities	$ 3,619,174	$ 3,662,721
Estimate of Fair Value Measurement [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated fair value of trading securities	3,777,848	3,844,754
U.S. Government and Agency [Member] | Estimate of Fair Value Measurement [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated fair value of trading securities	401,584	439,946
Non-U.S. Government [Member] | Estimate of Fair Value Measurement [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated fair value of trading securities	444,279	476,224
Corporate [Member] | Estimate of Fair Value Measurement [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated fair value of trading securities	2,109,014	2,123,675
Municipal [Member] | Estimate of Fair Value Measurement [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated fair value of trading securities	34,545	41,034
Residential Mortgage-Backed [Member] | Estimate of Fair Value Measurement [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated fair value of trading securities	233,348	218,457
Commercial Mortgage-Backed [Member] | Estimate of Fair Value Measurement [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated fair value of trading securities	127,664	114,637
Asset-Backed [Member] | Estimate of Fair Value Measurement [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated fair value of trading securities	268,740	248,748
Total Fixed Maturity and Short-term Investments [Member] | Estimate of Fair Value Measurement [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated fair value of trading securities	3,619,174	3,662,721
Equities - U.S. [Member] | Estimate of Fair Value Measurement [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated fair value of trading securities	107,341	115,285
Equities - International [Member] | Estimate of Fair Value Measurement [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated fair value of trading securities	$ 51,333	$ 66,748