Schedule I - Summary of Investments Other Than Investments in Related Parties (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 6,381,559
Fair Value	7,034,031
Amount at which shown in the balance sheet	7,058,630
Equities - U.S.
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	0
Fair Value	94,988
Amount at which shown in the balance sheet	94,988
Other investments, at fair value
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	0
Fair Value	659,261
Amount at which shown in the balance sheet	659,261
Fair value of other investments in funds or companies owned by or affiliated with related parties	374,800
Other investments, at cost
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	133,071
Fair Value	130,268
Amount at which shown in the balance sheet	133,071
Trading Securities | Fixed Maturity Securities and Short Term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	5,148,833
Fair Value	5,078,144
Amount at which shown in the balance sheet	5,078,144
Trading Securities | Fixed Maturity Securities and Short Term Investments | U.S. Government and Agency
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	751,512
Fair Value	750,957
Amount at which shown in the balance sheet	750,957
Trading Securities | Fixed Maturity Securities and Short Term Investments | Non-U.S. Government
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	367,887
Fair Value	359,002
Amount at which shown in the balance sheet	359,002
Trading Securities | Fixed Maturity Securities and Short Term Investments | Corporate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,667,290
Fair Value	2,631,682
Amount at which shown in the balance sheet	2,631,682
Trading Securities | Fixed Maturity Securities and Short Term Investments | Municipal Bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	22,364
Fair Value	22,247
Amount at which shown in the balance sheet	22,247
Trading Securities | Fixed Maturity Securities and Short Term Investments | Residential Mortgage-Backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	394,010
Fair Value	391,247
Amount at which shown in the balance sheet	391,247
Trading Securities | Fixed Maturity Securities and Short Term Investments | Commercial Mortgage-Backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	286,373
Fair Value	284,575
Amount at which shown in the balance sheet	284,575
Trading Securities | Fixed Maturity Securities and Short Term Investments | Asset-Backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	659,396
Fair Value	638,434
Amount at which shown in the balance sheet	638,434
Held-to-maturity Securities | Fixed Maturity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	790,866
Fair Value	769,069
Amount at which shown in the balance sheet	790,866
Held-to-maturity Securities | Fixed Maturity Securities | U.S. Government and Agency
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	19,771
Fair Value	19,321
Amount at which shown in the balance sheet	19,771
Held-to-maturity Securities | Fixed Maturity Securities | Non-U.S. Government
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	40,503
Fair Value	39,058
Amount at which shown in the balance sheet	40,503
Held-to-maturity Securities | Fixed Maturity Securities | Corporate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	730,592
Fair Value	710,692
Amount at which shown in the balance sheet	730,592
Available-for-sale Securities | Fixed Maturity Securities and Short Term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	308,789
Fair Value	302,301
Amount at which shown in the balance sheet	302,301
Available-for-sale Securities | Fixed Maturity Securities and Short Term Investments | U.S. Government and Agency
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	25,102
Fair Value	24,841
Amount at which shown in the balance sheet	24,841
Available-for-sale Securities | Fixed Maturity Securities and Short Term Investments | Non-U.S. Government
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	89,631
Fair Value	85,784
Amount at which shown in the balance sheet	85,784
Available-for-sale Securities | Fixed Maturity Securities and Short Term Investments | Corporate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	182,773
Fair Value	180,384
Amount at which shown in the balance sheet	180,384
Available-for-sale Securities | Fixed Maturity Securities and Short Term Investments | Municipal Bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	5,959
Fair Value	5,927
Amount at which shown in the balance sheet	5,927
Available-for-sale Securities | Fixed Maturity Securities and Short Term Investments | Residential Mortgage-Backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	665
Fair Value	715
Amount at which shown in the balance sheet	715
Available-for-sale Securities | Fixed Maturity Securities and Short Term Investments | Asset-Backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	4,660
Fair Value	4,650
Amount at which shown in the balance sheet	4,650
Registered Investment Companies
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair value of the investments in funds	21,000	$ 25,600
Registered Investment Companies | Equities - U.S.
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair value of the investments in funds	$ 21,000