Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
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	13
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	13
Exchange
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	374
Exchange | Private equity
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	143
Exchange | Mezzanine debt
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	145
Exchange | Real estate
Contractual commitments
Maximum contractual commitment to invest in limited partnerships	$ 86