Investments - Other Investments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investment Holdings [Line Items]
Other investments recorded in private equities	$ 389,728	$ 192,264
Private equity funds [Member]
Investment Holdings [Line Items]
Other investments recorded in private equities	115,199	107,388
Bond funds [Member]
Investment Holdings [Line Items]
Other investments recorded in private equities	147,609	54,537
Fixed income hedge funds [Member]
Investment Holdings [Line Items]
Other investments recorded in private equities	52,322	24,395
Equity funds [Member]
Investment Holdings [Line Items]
Other investments recorded in private equities	53,889
Real estate debt fund [Member]
Investment Holdings [Line Items]
Other investments recorded in private equities	15,861
Other [Member]
Investment Holdings [Line Items]
Other investments recorded in private equities	$ 4,848	$ 5,944