Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012
Indemnity
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	$ 36
Indemnity | Private equity
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	16
Indemnity | Mezzanine debt
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	10
Indemnity | Real estate
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	10
Exchange
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	338
Exchange | Private equity
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	144
Exchange | Mezzanine debt
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	110
Exchange | Real estate
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	$ 84