Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Indemnity
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	$ 40
Indemnity | Private equity
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	17
Indemnity | Mezzanine debt
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	11
Indemnity | Real estate
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	12
Exchange
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	384
Exchange | Private equity
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	162
Exchange | Mezzanine debt
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	118
Exchange | Real estate
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	$ 104