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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 | Sep. 30, 2012 | Dec. 31, 2011
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Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | $ 4,887,684 | $ 5,346,897
Other investments amortized cost basis | 100 | 100
Investment other than available for sale securities pledged as collateral amortized cost basis | 5,600,921 | 6,130,012
Amortized cost basis of securities pledged as collateral | 285,284 | 261,958
Amortized Cost on Investment | 5,886,205 | 6,391,970
Gross Unrealized Gains | 648,340 | 627,588
Other | 100 | 100
Investment other than available for sale securities pledged as collateral gross unrealized gain | 648,670 | 627,645
Amortized cost basis of securities pledged as collateral | 581 | 1,696
Gross Unrealized Gains On Investment | 649,251 | 629,341
Gross Unrealized Losses | 95,334 | 144,196
Other Investment, Gross Unrealized Losses |  |
Investment other than available for sale securities pledged as collateral gross unrealized losses | 95,334 | 144,197
Gross unrealized loss on securities pledged as collateral | 5 | 124
Gross Unrealized Loss On Investment | 95,339 | 144,321
Estimated Fair Value | 5,440,690 | 5,830,289
Other Short-term Investments | 713,467 | 783,071
Other Investments | 100 | 100
Investment other than available for sale securities pledged as collateral | 6,154,257 | 6,613,460
Fair value of securities pledged as collateral | 285,860 | 263,530
Estimated fair value on investments | 6,440,117 | 6,876,990
Non - credit other - than - temporary Impairments | 9,463 | 20,907
Short-term Investments [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Other short-term investments amortized cost basis | 713,137 | 783,015
Short-term | 330 | 57
Other short-term investments gross unrealized losses |  | 1
Other Short-term Investments | 713,467 | 783,071
Municipal Obligations [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 1,680,404 | 1,858,493
Gross Unrealized Gains | 181,472 | 144,989
Gross Unrealized Losses | 102 | 483
Estimated Fair Value | 1,861,774 | 2,002,999
Corporate Obligations [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 1,039,248 | 1,087,629
Gross Unrealized Gains | 92,706 | 63,074
Gross Unrealized Losses | 9,870 | 23,203
Estimated Fair Value | 1,122,084 | 1,127,500
Foreign Obligations [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 66,947 | 89,951
Gross Unrealized Gains | 3,448 | 4,844
Estimated Fair Value | 70,395 | 94,795
US Government Obligations [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 110,945 | 107,717
Amortized cost basis of securities pledged as collateral | 285,284 | 259,401
Gross Unrealized Gains | 1,071 | 3,847
Amortized cost basis of securities pledged as collateral | 581 | 1,525
Gross Unrealized Losses | 430 | 2
Gross unrealized loss on securities pledged as collateral | 5 | 124
Estimated Fair Value | 111,586 | 111,562
Fair value of securities pledged as collateral | 285,860 | 260,802
U.S. Agency Obligations [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 79,999 | 80,960
Gross Unrealized Gains | 4,024 | 5,911
Estimated Fair Value | 84,023 | 86,871
Residential Mortgage-backed securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 1,144,258 | 1,119,517
Amortized cost basis of securities pledged as collateral |  | 2,557
Gross Unrealized Gains | 301,074 | 350,816
Amortized cost basis of securities pledged as collateral |  | 171
Gross Unrealized Losses | 36,306 | 57,816
Estimated Fair Value | 1,409,026 | 1,412,517
Fair value of securities pledged as collateral |  | 2,728
Non - credit other - than - temporary Impairments | 9,463 | 20,907
Collateralized debt obligations [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 36,814 | 48,686
Gross Unrealized Gains | 873 | 142
Gross Unrealized Losses | 878 | 2,591
Estimated Fair Value | 36,809 | 46,237
Other Asset-backed Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 729,069 | 953,944
Gross Unrealized Gains | 63,672 | 53,965
Gross Unrealized Losses | 47,748 | 60,101
Estimated Fair Value | $ 744,993 | $ 947,808

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