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Investments - Summary of Amortized Cost and Estimated Fair Value of Investments, Excluding VIE Investments (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 4,751,824 $ 5,346,897
Other investments amortized cost basis 100 100
Investment other than available for sale securities pledged as collateral amortized cost basis 5,413,143 6,130,012
Amortized cost basis of securities pledged as collateral 265,517 261,958
Amortized Cost on Investment 5,678,660 6,391,970
Gross Unrealized Gains 728,286 627,588
Other 100 100
Investment other than available for sale securities pledged as collateral gross unrealized gain 728,725 627,645
Amortized cost basis of securities pledged as collateral 262 1,696
Gross Unrealized Gains On Investment 728,987 629,341
Gross Unrealized Losses 77,715 144,196
Other      
Other Investment, Gross Unrealized Gain Losses      
Investment other than available for sale securities pledged as collateral gross unrealized losses 77,715 144,197
Gross unrealized loss on securities pledged as collateral    124
Gross Unrealized Loss On Investment 77,715 144,321
Estimated Fair Value 5,402,395 5,830,289
Other Short-term Investments 661,658 783,071
Other Investments 100 100
Investment other than available for sale securities pledged as collateral 6,064,153 6,613,460
Fair value of securities pledged as collateral 265,779 263,530
Total investments 6,329,932 6,876,990
Non - credit other - than - temporary Impairments 6,892 20,907
Other      
Non - credit other - than - temporary Impairments 6,892 20,907
Non - credit other - than - temporary Impairments 6,892 20,907
Municipal obligations [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,632,637 1,858,493
Gross Unrealized Gains 182,476 144,989
Gross Unrealized Losses 383 483
Estimated Fair Value 1,814,730 2,002,999
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
Corporate obligations [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,029,041 1,087,629
Gross Unrealized Gains 92,250 63,074
Gross Unrealized Losses 9,117 23,203
Estimated Fair Value 1,112,174 1,127,500
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
Foreign obligations [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 67,347 89,951
Gross Unrealized Gains 2,765 4,844
Gross Unrealized Losses      
Estimated Fair Value 70,112 94,795
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
U.S. government obligations [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 127,037 107,717
Amortized cost basis of securities pledged as collateral 265,517 259,401
Gross Unrealized Gains 872 3,847
Amortized cost basis of securities pledged as collateral 262 1,525
Gross Unrealized Losses 626 2
Gross unrealized loss on securities pledged as collateral    124
Estimated Fair Value 127,283 111,562
Fair value of securities pledged as collateral 265,779 260,802
Non - credit other - than - temporary Impairments      
Non-credit other-than-temporary Impairments      
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
U.S. agency obligations [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 79,295 80,960
Gross Unrealized Gains 3,240 5,911
Gross Unrealized Losses      
Estimated Fair Value 82,535 86,871
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
Residential mortgage-backed securities [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,096,202 1,119,517
Amortized cost basis of securities pledged as collateral   2,557
Amortized Cost on Investment 1,096,202  
Gross Unrealized Gains 379,935 350,816
Amortized cost basis of securities pledged as collateral    171
Gross Unrealized Losses 20,555 57,816
Gross unrealized loss on securities pledged as collateral      
Estimated Fair Value 1,455,582 1,412,517
Fair value of securities pledged as collateral   2,728
Total investments 1,455,582  
Non - credit other - than - temporary Impairments 6,892 20,907
Non-credit other-than-temporary Impairments      
Non - credit other - than - temporary Impairments 6,892 20,907
Non - credit other - than - temporary Impairments 6,892 20,907
Collateralized debt obligations [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 32,855 48,686
Amortized Cost on Investment 32,855  
Gross Unrealized Gains 1,015 142
Gross Unrealized Losses 528 2,591
Estimated Fair Value 33,342 46,237
Total investments 33,342  
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
Asset-backed Securities [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 687,410 953,944
Gross Unrealized Gains 65,733 53,965
Gross Unrealized Losses 46,506 60,101
Estimated Fair Value 706,637 947,808
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
Short-term [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Other short-term investments amortized cost basis 661,219 783,015
Short-term 439 57
Other short-term investments gross unrealized losses    1
Gross Unrealized Loss On Investment   1
Other Short-term Investments 661,658 783,071
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
Non - credit other - than - temporary Impairments      
Collateralized Investments [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Non-credit other-than-temporary Impairments