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Schedule - I - Summary of Investments Other Than Investments in Related Parties (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Amortized Cost $ 5,678,660
Estimated Fair Value 6,329,932
Amount at which shown in the balance sheet 6,329,932
U.S. government obligations [Member]
  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Amortized Cost 392,554
Estimated Fair Value 393,062
Amount at which shown in the balance sheet 393,062
U.S. agency obligations [Member]
  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Amortized Cost 79,295
Estimated Fair Value 82,535
Amount at which shown in the balance sheet 82,535
Municipal obligations [Member]
  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Amortized Cost 1,632,637
Estimated Fair Value 1,814,730
Amount at which shown in the balance sheet 1,814,730
Residential mortgage-backed securities [Member]
  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Amortized Cost 1,096,202
Estimated Fair Value 1,455,582
Amount at which shown in the balance sheet 1,455,582
Collateralized debt obligations [Member]
  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Amortized Cost 32,855
Estimated Fair Value 33,342
Amount at which shown in the balance sheet 33,342
Other asset-backed securities [Member]
  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Amortized Cost 687,410
Estimated Fair Value 706,637
Amount at which shown in the balance sheet 706,637
Corporate obligations [Member]
  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Amortized Cost 1,029,041
Estimated Fair Value 1,112,174
Amount at which shown in the balance sheet 1,112,174
Foreign obligations [Member]
  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Amortized Cost 67,347
Estimated Fair Value 70,112
Amount at which shown in the balance sheet 70,112
Short-term [Member]
  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Amortized Cost 661,219
Estimated Fair Value 661,658
Amount at which shown in the balance sheet 661,658
Other [Member]
  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Amortized Cost 100
Estimated Fair Value 100
Amount at which shown in the balance sheet $ 100