Investments - Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Sep. 30, 2011
Available-for-sale Securities, Amortized Cost Basis	$ 15,278,553	$ 15,236,307
Available-for-sale Securities, Gross Unrealized Gains	1,088,939	653,005
Available-for-sale Securities, Gross Unrealized Losses	(30,492)	(234,795)
Available-for-sale Securities, Fair Value and Carrying Value	16,337,000	15,654,517
Derivative investments, Amortized Cost	142,123	171,612
Derivative investments, Gross Unrealized Gains	66,973	405
Derivative investments, Gross Unrealized Losses	(8,429)	(119,682)
Derivative investments, Fair Value and Carrying Value	200,667	52,335
Asset-backed loans and other invested assets, Amortized Cost	198,868	44,279
Asset-backed loans and other invested assets, Gross Unrealized Gains
Asset-backed loans and other invested assets, Gross Unrealized Losses
Asset-backed loans and other invested assets	198,868	44,279
Total investments, Amortized Cost	15,619,544	15,452,198
Total investment, Gross Unrealized Gains	1,155,912	653,410
Total investment, Gross Unrealized Losses	(38,921)	(354,477)
Total investments	16,736,535	15,751,131
Asset-backed securities [Member]
Available-for-sale Securities, Amortized Cost Basis	1,010,938	501,469
Available-for-sale Securities, Gross Unrealized Gains	18,553	1,785
Available-for-sale Securities, Gross Unrealized Losses	(1,609)	(2,770)
Available-for-sale Securities, Fair Value and Carrying Value	1,027,882	500,484
Commercial mortgage-backed securities [Member]
Available-for-sale Securities, Amortized Cost Basis	520,043	580,313
Available-for-sale Securities, Gross Unrealized Gains	36,178	3,427
Available-for-sale Securities, Gross Unrealized Losses	(2,407)	(18,163)
Available-for-sale Securities, Fair Value and Carrying Value	553,814	565,577
Corporates [Member]
Available-for-sale Securities, Amortized Cost Basis	10,211,804	11,479,862
Available-for-sale Securities, Gross Unrealized Gains	807,175	506,264
Available-for-sale Securities, Gross Unrealized Losses	(9,968)	(130,352)
Available-for-sale Securities, Fair Value and Carrying Value	11,009,011	11,855,774
Equity securities [Member]
Available-for-sale Securities, Amortized Cost Basis	237,499	292,112
Available-for-sale Securities, Gross Unrealized Gains	11,860	3,964
Available-for-sale Securities, Gross Unrealized Losses	(1,272)	(9,033)
Available-for-sale Securities, Fair Value and Carrying Value	248,087	287,043
Hybrids [Member]
Available-for-sale Securities, Amortized Cost Basis	519,009	699,915
Available-for-sale Securities, Gross Unrealized Gains	18,836	10,429
Available-for-sale Securities, Gross Unrealized Losses	(9,550)	(51,055)
Available-for-sale Securities, Fair Value and Carrying Value	528,295	659,289
Municipals [Member]
Available-for-sale Securities, Amortized Cost Basis	1,083,231	824,562
Available-for-sale Securities, Gross Unrealized Gains	141,854	111,929
Available-for-sale Securities, Gross Unrealized Losses	(1,090)	(7)
Available-for-sale Securities, Fair Value and Carrying Value	1,223,995	936,484
Agency residential mortgage-backed securities [Member]
Available-for-sale Securities, Amortized Cost Basis	149,455	217,354
Available-for-sale Securities, Gross Unrealized Gains	5,769	4,966
Available-for-sale Securities, Gross Unrealized Losses	(334)	(295)
Available-for-sale Securities, Fair Value and Carrying Value	154,890	222,025
Non-agency residential mortgage-backed securities [Member]
Available-for-sale Securities, Amortized Cost Basis	629,122	465,666
Available-for-sale Securities, Gross Unrealized Gains	35,799	1,971
Available-for-sale Securities, Gross Unrealized Losses	(4,262)	(23,120)
Available-for-sale Securities, Fair Value and Carrying Value	660,659	444,517
U.S. Government Securities [Member]
Available-for-sale Securities, Amortized Cost Basis	917,452	175,054
Available-for-sale Securities, Gross Unrealized Gains	12,915	8,270
Available-for-sale Securities, Fair Value and Carrying Value	$ 930,367	$ 183,324

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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Other investments not otherwise specified in the taxonomy.

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