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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
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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 |  |

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