Investments - Investment Holdings by Type of Instrument (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	$ 530,650	$ 476,972
Investment Owned, at Fair Value	526,405	469,747
Investment Owned, at Carrying Value	526,405	469,747
Categories of Investments, Marketable Securities, Trading Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost		96,571
Investment Owned, at Fair Value		89,346
Investment Owned, at Carrying Value		89,346
Categories of Investments, Marketable Securities, Available-for-Sale Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	347,140
Investment Owned, at Fair Value	342,895
Investment Owned, at Carrying Value	342,895
Cash and Cash Equivalents [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	180,857	347,392
Investment Owned, at Fair Value	180,857	347,392
Investment Owned, at Carrying Value	180,857	347,392
Restricted Cash and Cash Equivalents [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	2,653	33,009
Investment Owned, at Fair Value	2,653	33,009
Investment Owned, at Carrying Value	2,653	33,009
U.S. Government Obligations and Agencies [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost		3,192
Investment Owned, at Fair Value		4,009
Investment Owned, at Carrying Value		4,009
U.S. Government Obligations and Agencies [Member] | Categories of Investments, Marketable Securities, Available-for-Sale Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	105,235
Investment Owned, at Fair Value	104,154
Investment Owned, at Carrying Value	104,154
Equity Securities, Common Stock, Metals and Mining [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost		31,113
Investment Owned, at Fair Value		26,130
Investment Owned, at Carrying Value		26,130
Equity Securities, Common Stock, Energy [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost		12,053
Investment Owned, at Fair Value		10,868
Investment Owned, at Carrying Value		10,868
Equity Securities, Common Stock, Other [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost		8,416
Investment Owned, at Fair Value		8,215
Investment Owned, at Carrying Value		8,215
Equity Securities, Exchange-Traded and Mutual Funds, Metals and Mining [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost		22,687
Investment Owned, at Fair Value		21,989
Investment Owned, at Carrying Value		21,989
Equity Securities, Exchange-Traded and Mutual Funds, Agriculture [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost		10,705
Investment Owned, at Fair Value		10,265
Investment Owned, at Carrying Value		10,265
Equity Securities, Exchange-Traded and Mutual Funds, Energy [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost		4,992
Investment Owned, at Fair Value		5,068
Investment Owned, at Carrying Value		5,068
Equity Securities, Exchange-Traded and Mutual Funds, Indices [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost		2,827
Investment Owned, at Fair Value		2,506
Investment Owned, at Carrying Value		2,506
Non-Hedging Derivative Asset (Liability), Net [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost		69
Investment Owned, at Fair Value		(21)
Investment Owned, at Carrying Value		(21)
Other Investments [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost		517
Investment Owned, at Fair Value		317
Investment Owned, at Carrying Value		317
Corporate Bonds [Member] | Categories of Investments, Marketable Securities, Available-for-Sale Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	95,437
Investment Owned, at Fair Value	93,971
Investment Owned, at Carrying Value	93,971
Mortgage-Backed and Asset-Backed Securities [Member] | Categories of Investments, Marketable Securities, Available-for-Sale Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	91,544
Investment Owned, at Fair Value	91,263
Investment Owned, at Carrying Value	91,263
Common Stock [Member] | Categories of Investments, Marketable Securities, Available-for-Sale Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	11,999
Investment Owned, at Fair Value	11,625
Investment Owned, at Carrying Value	11,625
Mutual Funds [Member] | Categories of Investments, Marketable Securities, Available-for-Sale Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	42,925
Investment Owned, at Fair Value	41,882
Investment Owned, at Carrying Value	$ 41,882