Investments - Investment Holdings by Type of Instrument (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	$ 474,927	$ 476,972
Investment Owned, at Fair Value	474,315	469,747
Investment Owned, at Carrying Value	474,315	469,747
Cash and Cash Equivalents [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	117,275	347,392
Investment Owned, at Fair Value	117,275	347,392
Investment Owned, at Carrying Value	117,275	347,392
Restricted Cash and Cash Equivalents [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	2,600	33,009
Investment Owned, at Fair Value	2,600	33,009
Investment Owned, at Carrying Value	2,600	33,009
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, Trading Securities [Member] | U.S. Government Obligations and Agencies [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
Categories of Investments, Marketable Securities, Trading Securities [Member] | Common Stock [Member] | Metals and Mining [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
Categories of Investments, Marketable Securities, Trading Securities [Member] | Common Stock [Member] | Energy [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
Categories of Investments, Marketable Securities, Trading Securities [Member] | Common Stock [Member] | Other [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
Categories of Investments, Marketable Securities, Trading Securities [Member] | Exchange Traded and Mutual Funds [Member] | Metals and Mining [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
Categories of Investments, Marketable Securities, Trading Securities [Member] | Exchange Traded and Mutual Funds [Member] | Energy [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
Categories of Investments, Marketable Securities, Trading Securities [Member] | Exchange Traded and Mutual Funds [Member] | Agriculture [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
Categories of Investments, Marketable Securities, Trading Securities [Member] | Exchange Traded and Mutual Funds [Member] | Indices [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
Categories of Investments, Marketable Securities, Trading Securities [Member] | Non - Hedging Derivative Asset (Liability), Net [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)
Categories of Investments, Marketable Securities, Trading Securities [Member] | Other Investments [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
Categories of Investments, Marketable Securities, Available-for-Sale Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	355,052
Investment Owned, at Fair Value	354,440
Investment Owned, at Carrying Value	354,440
Categories of Investments, Marketable Securities, Available-for-Sale Securities [Member] | U.S. Government Obligations and Agencies [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	105,229
Investment Owned, at Fair Value	104,215
Investment Owned, at Carrying Value	104,215
Categories of Investments, Marketable Securities, Available-for-Sale Securities [Member] | Common Stock [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	8,500
Investment Owned, at Fair Value	9,295
Investment Owned, at Carrying Value	9,295
Categories of Investments, Marketable Securities, Available-for-Sale Securities [Member] | Corporate Bonds [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	94,708
Investment Owned, at Fair Value	94,203
Investment Owned, at Carrying Value	94,203
Categories of Investments, Marketable Securities, Available-for-Sale Securities [Member] | Mortgage-Backed and Asset-Backed Securities [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	91,502
Investment Owned, at Fair Value	91,000
Investment Owned, at Carrying Value	91,000
Categories of Investments, Marketable Securities, Available-for-Sale Securities [Member] | Mutual Funds [Member]
Investment Holdings [Line Items]
Investment Owned, at Cost or Amortized Cost	55,113
Investment Owned, at Fair Value	55,727
Investment Owned, at Carrying Value	$ 55,727