Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012
Indemnity
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	$ 39
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	10
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	373
Exchange | Private equity
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	159
Exchange | Mezzanine debt
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	119
Exchange | Real estate
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	$ 95