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Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 4,657,096 $ 4,751,824
Investment other than available for sale securities pledged as collateral amortized cost basis 5,425,015 5,413,043
Amortized cost basis of securities pledged as collateral 228,066 265,517
Amortized Cost available-for-sale on Investment 5,653,081 5,678,560
Gross Unrealized Gains 812,059 728,286
Investment other than available for sale securities pledged as collateral gross unrealized gain 812,072 728,725
Gross unrealized gains on securities pledged as collateral 162 262
Gross Unrealized Gains On available-for-sale Investment 812,234 728,987
Gross Unrealized Losses 66,865 77,715
Investment other than available for sale securities pledged as collateral gross unrealized losses 66,865 77,715
Gross unrealized losses on securities pledged as collateral      
Gross Unrealized Loss On available-for-sale Investment 66,865 77,715
Estimated Fair Value 5,402,290 5,402,395
Short-term Investments 767,932 661,658
Investment other than available for sale securities pledged as collateral 6,170,222 6,064,053
Fair value of securities pledged as collateral 228,228 265,779
Total available-for-sale investments 6,398,450 6,329,832
Non - credit other - than - temporary Impairments 6,590 6,892
Non - credit other - than - temporary Impairments 6,590 6,892
Non - credit other - than - temporary Impairments 6,590 6,892
Municipal obligations [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,606,136 1,662,124
Gross Unrealized Gains 188,861 187,191
Gross Unrealized Losses 642 383
Estimated Fair Value 1,794,355 1,848,932
Corporate obligations [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 986,866 999,554
Gross Unrealized Gains 89,231 87,535
Gross Unrealized Losses 7,024 9,117
Estimated Fair Value 1,069,073 1,077,972
Foreign obligations [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 10,796 67,347
Gross Unrealized Gains 1,082 2,765
Estimated Fair Value 11,878 70,112
U.S. government obligations [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 96,526 127,037
Amortized cost basis of securities pledged as collateral 228,066 265,517
Gross Unrealized Gains 1,786 872
Gross unrealized gains on securities pledged as collateral 162 262
Gross Unrealized Losses   626
Gross unrealized losses on securities pledged as collateral      
Estimated Fair Value 98,312 127,283
Fair value of securities pledged as collateral 228,228 265,779
Non - credit other - than - temporary Impairments      
U.S. agency obligations [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 51,255 79,295
Gross Unrealized Gains 2,720 3,240
Estimated Fair Value 53,975 82,535
Residential mortgage-backed securities [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,085,177 1,096,202
Amortized Cost available-for-sale on Investment 1,085,177  
Gross Unrealized Gains 466,111 379,935
Gross Unrealized Losses 14,399 20,555
Estimated Fair Value 1,536,889 1,455,582
Total available-for-sale investments 1,536,889  
Non - credit other - than - temporary Impairments 6,590 6,892
Non - credit other - than - temporary Impairments 6,590 6,892
Non - credit other - than - temporary Impairments 6,590 6,892
Collateralized debt obligations [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 26,598 32,855
Amortized Cost available-for-sale on Investment 26,598  
Gross Unrealized Gains 983 1,015
Gross Unrealized Losses 224 528
Estimated Fair Value 27,357 33,342
Total available-for-sale investments 27,357  
Asset-backed Securities [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 793,742 687,410
Gross Unrealized Gains 61,285 65,733
Gross Unrealized Losses 44,576 46,506
Estimated Fair Value 810,451 706,637
Short-term [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Equity securities, trading amortized cost basis 767,919 661,219
Short-term 13 439
Other short-term investments gross unrealized losses      
Short-term Investments 767,932 661,658
Collateralized Investments [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Non - credit other - than - temporary Impairments