Derivative Financial Instruments - Netting Arrangements (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets
Gross Amounts Recognized	$ 220,647	$ 169,423
Gross Amounts Offset in the Statements of Financial Condition	41,146	24,449
Net Amounts Presented in the Statements of Financial Condition	179,501	144,974
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	0
Net Amount	179,501	144,974
Liabilities
Gross Amounts Recognized	194,723	139,811
Gross Amounts Offset in the Statements of Financial Condition	42,830	24,449
Net Amounts Presented in the Statements of Financial Condition	151,893	115,362
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	0	17,359
Net Amount	151,893	98,003
Listed options
Assets
Gross Amounts Recognized	178,360	144,586
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	178,360	144,586
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	0
Net Amount	178,360	144,586
Liabilities
Gross Amounts Recognized	151,893	115,362
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	151,893	115,362
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	0	17,359
Net Amount	151,893	98,003
Foreign currency forward contracts
Assets
Gross Amounts Recognized	445	60
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	445	60
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	0
Net Amount	445	60
Futures
Assets
Gross Amounts Recognized	41,842	24,679
Gross Amounts Offset in the Statements of Financial Condition	41,146	24,436
Net Amounts Presented in the Statements of Financial Condition	696	243
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	0
Net Amount	696	243
Liabilities
Gross Amounts Recognized	42,549	24,436
Gross Amounts Offset in the Statements of Financial Condition	42,549	24,436
Net Amounts Presented in the Statements of Financial Condition	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	0	0
Net Amount	0	0
Amount offset against liabilities to maintain margin in excess of the net liability	6,100
Amount offset against margin	36,400
Swaps
Assets
Gross Amounts Recognized		98
Gross Amounts Offset in the Statements of Financial Condition		13
Net Amounts Presented in the Statements of Financial Condition		85
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets		0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets		0
Net Amount		85
Liabilities
Gross Amounts Recognized	281	13
Gross Amounts Offset in the Statements of Financial Condition	281	13
Net Amounts Presented in the Statements of Financial Condition	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	0	0
Net Amount	$ 0	$ 0