Fair Value Measurements - Categorization of Investments and Certain Other Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	$ 251,932	$ 97,852
Corporate Bonds, Municipal Bonds and Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	168,077	44,648
Securities Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	9,099	6,131
Investment Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	52,919	27,699
Financial Instruments Owned and Pledged as Collateral at Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	21,837	19,374
Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	201,675	51,409
Cash and Cash Equivalents	273,984	596,141
Level I [Member] | Corporate Bonds, Municipal Bonds and Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	119,600	0
Level I [Member] | Securities Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	7,319	4,336
Level I [Member] | Investment Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	52,919	27,699
Level I [Member] | Financial Instruments Owned and Pledged as Collateral at Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	21,837	19,374
Level II [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	50,257	46,443
Cash and Cash Equivalents	0	0
Level II [Member] | Corporate Bonds, Municipal Bonds and Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	48,477	44,648
Level II [Member] | Securities Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	1,780	1,795
Level II [Member] | Investment Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	0	0
Level II [Member] | Financial Instruments Owned and Pledged as Collateral at Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	0	0
Level III [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	0	0
Cash and Cash Equivalents	0	0
Level III [Member] | Corporate Bonds, Municipal Bonds and Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	0	0
Level III [Member] | Securities Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	0	0
Level III [Member] | Investment Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	0	0
Level III [Member] | Financial Instruments Owned and Pledged as Collateral at Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	0	0
Treasury Bills, Municipal Bonds and Commercial Paper [Member] | Corporate Bonds, Municipal Bonds and Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents	130,555	10,354
Treasury Bills, Municipal Bonds and Commercial Paper [Member] | Securities Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents	$ 3,190	$ 3,092