Fair Value (Details 4) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair value by pricing source
Available-for-sale securities	$ 9,304	$ 9,159
Indemnity
Fair value by pricing source
Available-for-sale securities	470	507
Exchange
Fair value by pricing source
Available-for-sale securities	8,834	8,652
Fair value measurements on a recurring basis
Fair value by pricing source
Total	12,098	11,704
Fair value measurements on a recurring basis \| Indemnity
Fair value by pricing source
Available-for-sale securities	470	507
Total	490	526
Fair value measurements on a recurring basis \| Indemnity \| Fixed maturities
Fair value by pricing source
Available-for-sale securities	421	452
Fair value measurements on a recurring basis \| Indemnity \| Fixed maturities \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	404
Fair value measurements on a recurring basis \| Indemnity \| Fixed maturities \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	15
Fair value measurements on a recurring basis \| Indemnity \| Fixed maturities \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	2
Fair value measurements on a recurring basis \| Indemnity \| Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	23	29
Fair value measurements on a recurring basis \| Indemnity \| Nonredeemable preferred stock \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	21
Fair value measurements on a recurring basis \| Indemnity \| Nonredeemable preferred stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	2
Fair value measurements on a recurring basis \| Indemnity \| Nonredeemable preferred stock \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Indemnity \| Common stock
Fair value by pricing source
Available-for-sale securities	26	26
Fair value measurements on a recurring basis \| Indemnity \| Common stock \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	26
Fair value measurements on a recurring basis \| Indemnity \| Common stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Indemnity \| Common stock \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Indemnity \| Real estate
Fair value by pricing source
Other investments	20	19
Fair value measurements on a recurring basis \| Indemnity \| Real estate \| Priced via unobservable inputs
Fair value by pricing source
Other investments	20
Fair value measurements on a recurring basis \| Exchange
Fair value by pricing source
Available-for-sale securities	8,834	8,652
Total	11,608	11,178
Fair value measurements on a recurring basis \| Exchange \| Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,891	7,707
Fair value measurements on a recurring basis \| Exchange \| Fixed maturities \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	7,724
Fair value measurements on a recurring basis \| Exchange \| Fixed maturities \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	147
Fair value measurements on a recurring basis \| Exchange \| Fixed maturities \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	20
Fair value measurements on a recurring basis \| Exchange \| Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	631	631
Fair value measurements on a recurring basis \| Exchange \| Nonredeemable preferred stock \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	604
Fair value measurements on a recurring basis \| Exchange \| Nonredeemable preferred stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	20
Fair value measurements on a recurring basis \| Exchange \| Nonredeemable preferred stock \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	7
Fair value measurements on a recurring basis \| Exchange \| Common stock
Fair value by pricing source
Available-for-sale securities	312	314
Trading and available-for-sale securities	2,974
Fair value measurements on a recurring basis \| Exchange \| Common stock \| Priced via pricing services
Fair value by pricing source
Trading and available-for-sale securities	2,967
Fair value measurements on a recurring basis \| Exchange \| Common stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis \| Exchange \| Common stock \| Priced via unobservable inputs
Fair value by pricing source
Trading and available-for-sale securities	7
Fair value measurements on a recurring basis \| Exchange \| Real estate
Fair value by pricing source
Other investments	112	109
Fair value measurements on a recurring basis \| Exchange \| Real estate \| Priced via unobservable inputs
Fair value by pricing source
Other investments	112
Fair value measurements on a recurring basis \| Level 1
Fair value by pricing source
Total	3,231	2,945
Fair value measurements on a recurring basis \| Level 1 \| Indemnity
Fair value by pricing source
Available-for-sale securities	28	30
Total	28	30
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Fixed maturities
Fair value by pricing source
Available-for-sale securities	0	0
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Fixed maturities \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Fixed maturities \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Fixed maturities \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	2	4
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Nonredeemable preferred stock \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	2
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Nonredeemable preferred stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Nonredeemable preferred stock \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Common stock
Fair value by pricing source
Available-for-sale securities	26	26
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Common stock \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	26
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Common stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Common stock \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Real estate
Fair value by pricing source
Other investments	0	0
Fair value measurements on a recurring basis \| Level 1 \| Indemnity \| Real estate \| Priced via unobservable inputs
Fair value by pricing source
Other investments	0
Fair value measurements on a recurring basis \| Level 1 \| Exchange
Fair value by pricing source
Available-for-sale securities	548	513
Total	3,203	2,915
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Fixed maturities
Fair value by pricing source
Available-for-sale securities	0	0
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Fixed maturities \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Fixed maturities \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Fixed maturities \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	236	199
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Nonredeemable preferred stock \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	236
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Nonredeemable preferred stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Nonredeemable preferred stock \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Common stock
Fair value by pricing source
Available-for-sale securities	312	314
Trading and available-for-sale securities	2,967
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Common stock \| Priced via pricing services
Fair value by pricing source
Trading and available-for-sale securities	2,967
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Common stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Common stock \| Priced via unobservable inputs
Fair value by pricing source
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Real estate
Fair value by pricing source
Other investments	0	0
Fair value measurements on a recurring basis \| Level 1 \| Exchange \| Real estate \| Priced via unobservable inputs
Fair value by pricing source
Other investments	0
Fair value measurements on a recurring basis \| Level 2
Fair value by pricing source
Total	8,636	8,553
Fair value measurements on a recurring basis \| Level 2 \| Indemnity
Fair value by pricing source
Available-for-sale securities	439	473
Total	439	473
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Fixed maturities
Fair value by pricing source
Available-for-sale securities	418	448
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Fixed maturities \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	404
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Fixed maturities \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	14
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Fixed maturities \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	21	25
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Nonredeemable preferred stock \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	19
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Nonredeemable preferred stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	2
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Nonredeemable preferred stock \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Common stock
Fair value by pricing source
Available-for-sale securities	0	0
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Common stock \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Common stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Common stock \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Real estate
Fair value by pricing source
Other investments	0	0
Fair value measurements on a recurring basis \| Level 2 \| Indemnity \| Real estate \| Priced via unobservable inputs
Fair value by pricing source
Other investments	0
Fair value measurements on a recurring basis \| Level 2 \| Exchange
Fair value by pricing source
Available-for-sale securities	8,197	8,080
Total	8,197	8,080
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Fixed maturities
Fair value by pricing source
Available-for-sale securities	7,814	7,648
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Fixed maturities \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	7,724
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Fixed maturities \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	90
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Fixed maturities \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	383	432
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Nonredeemable preferred stock \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	368
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Nonredeemable preferred stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	15
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Nonredeemable preferred stock \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Common stock
Fair value by pricing source
Available-for-sale securities	0	0
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Common stock \| Priced via pricing services
Fair value by pricing source
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Common stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Common stock \| Priced via unobservable inputs
Fair value by pricing source
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Real estate
Fair value by pricing source
Other investments	0	0
Fair value measurements on a recurring basis \| Level 2 \| Exchange \| Real estate \| Priced via unobservable inputs
Fair value by pricing source
Other investments	0
Fair value measurements on a recurring basis \| Level 3
Fair value by pricing source
Total	231	206
Fair value measurements on a recurring basis \| Level 3 \| Indemnity
Fair value by pricing source
Available-for-sale securities	3	4
Total	23	23
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Fixed maturities
Fair value by pricing source
Available-for-sale securities	3	4
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Fixed maturities \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Fixed maturities \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	1
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Fixed maturities \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	2
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	0	0
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Nonredeemable preferred stock \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Nonredeemable preferred stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Nonredeemable preferred stock \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Common stock
Fair value by pricing source
Available-for-sale securities	0	0
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Common stock \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Common stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Common stock \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Real estate
Fair value by pricing source
Other investments	20	19
Fair value measurements on a recurring basis \| Level 3 \| Indemnity \| Real estate \| Priced via unobservable inputs
Fair value by pricing source
Other investments	20
Fair value measurements on a recurring basis \| Level 3 \| Exchange
Fair value by pricing source
Available-for-sale securities	89	59
Total	208	183
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Fixed maturities
Fair value by pricing source
Available-for-sale securities	77	59
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Fixed maturities \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Fixed maturities \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	57
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Fixed maturities \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	20
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Nonredeemable preferred stock
Fair value by pricing source
Available-for-sale securities	12	0
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Nonredeemable preferred stock \| Priced via pricing services
Fair value by pricing source
Available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Nonredeemable preferred stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Available-for-sale securities	5
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Nonredeemable preferred stock \| Priced via unobservable inputs
Fair value by pricing source
Available-for-sale securities	7
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Common stock
Fair value by pricing source
Available-for-sale securities	0	0
Trading and available-for-sale securities	7
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Common stock \| Priced via pricing services
Fair value by pricing source
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Common stock \| Priced via market comparables/non-binding broker quote
Fair value by pricing source
Trading and available-for-sale securities	0
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Common stock \| Priced via unobservable inputs
Fair value by pricing source
Trading and available-for-sale securities	7
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Real estate
Fair value by pricing source
Other investments	112	109
Fair value measurements on a recurring basis \| Level 3 \| Exchange \| Real estate \| Priced via unobservable inputs
Fair value by pricing source
Other investments	112
Fair value measurements on a nonrecurring basis
Fair value by pricing source
Total	$ 0

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale and Trading Securities which consist of all investments in certain debt and equity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale and Trading.

+ References+ Details

- Definition

This element represents the total fair market value of investments in certain entities that calculate net asset value per share (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments). This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements, but may also be used in both the balance sheet and disclosure in the same submission. This item represents alternative investments as of the balance sheet date which may include investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds, and any other investment in which investee management calculates net asset value per share as the primary metric for investment value.

+ References+ Details

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

+ References+ Details

- Definition

+ References+ Details