Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Detail) (Cash And Securities Pledged Directly From The Investment Portfolio [Member], USD $)	Sep. 30, 2012	Dec. 31, 2011
Cash And Securities Pledged Directly From The Investment Portfolio [Member]
Fair Value of Cash and Underlying Securities	$ 735,229	$ 874,987
Fair Value of Cash and Securities Pledged to Investment and Repurchase Agreement Counterparties	432,592	564,036
Fair Value of Cash and Securities Pledged to Derivative Counterparties	$ 302,637	$ 310,951