Convertible Debt and Notes Payable (Details)			3 Months Ended	12 Months Ended				40 Months Ended
		Dec. 05, 2016	Dec. 31, 2017 USD ($) letters_of_credit	Dec. 31, 2017 USD ($) letters_of_credit	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jan. 31, 2014 USD ($)	Dec. 14, 2017 USD ($) $ / shares	Dec. 08, 2017 USD ($)	Jan. 31, 2016	Mar. 10, 2014 USD ($) $ / shares
Debt and Capital Lease Obligations [Line Items]
Stockholders' Equity Note, Stock Split, Conversion Ratio		0.25
Components of short-term borrowings and other debt
Convertible debt			$ 141,502,000	$ 141,502,000	$ 130,029,000
Note payable			132,970,000	132,970,000	60,953,000
Loans Payable to Bank			28,121,000	28,121,000	0
Other Notes Payable			8,247,000	8,247,000	14,237,000
Capital lease obligations			4,120,000	4,120,000	1,840,000
Debt, Long-term and Short-term, Combined Amount			314,960,000	314,960,000	207,059,000
Amortization of debt discount				7,435,000	6,885,000	$ 6,302,000
Cash convertible note hedge transaction				135,000,000	0	0
Purchase of treasury stock, cost				19,500,000			$ 300,000
Letter of credit, borrowing capacity			180,000,000	180,000,000
Capital Lease Obligations Incurred								$ 6,900,000
Capital Leases, Income Statement, Interest Expense				100,000	100,000	200,000
Future minimum lease payments for capital lease obligations
2017			2,059,000	2,059,000
2018			717,000	717,000
2019			639,000	639,000
2020			639,000	639,000
2021			420,000	420,000
Thereafter			0	0
Subtotal			4,474,000	4,474,000
Less: Amount representing interest	[1]		(354,000)	(354,000)
Capital lease obligations			$ 4,120,000	$ 4,120,000	1,840,000
Number of Letters of Credit | letters_of_credit			8	8
Minimum
Components of short-term borrowings and other debt
Capital Lease Obligation, Initial Term				48 months
Capital leases, interest rate			3.70%	3.70%
Maximum
Components of short-term borrowings and other debt
Capital Lease Obligation, Initial Term				60 months
Capital leases, interest rate			5.69%	5.69%
Letter of Credit, San Francisco Office, Expires January 2019 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			$ 710,157	$ 710,157
Letter of Credit, Connecticut Office, Expires January 2018 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			65,000	65,000
Letter of Credit, Boston Office, Expires March 2018 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			381,700	381,700
Letter of Credit, NY Office 1, Expires April 2018 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			354,939	354,939
Letter of Credit, NY Office 2, Expires May 2018 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			69,750	69,750
Letter of Credit, NY Office 3, Expires October 2018 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			595,540	595,540
Letter of Credit, NY Office 4, Expires October 2018 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			2,249,523	2,249,523
Letter of Credit, NY Office 5, Expires November 2018 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			1,600,000	1,600,000
Convertible Debt
Components of short-term borrowings and other debt
Convertible debt, unamortized discount			25,600,000	25,600,000	17,800,000
Repayments on long-term and short-term borrowings
2017			4,840,000	4,840,000
2018			38,925,000	38,925,000
2019			4,050,000	4,050,000
2020			4,050,000	4,050,000
2021			139,050,000	139,050,000
Thereafter			0	0
Subtotal			190,915,000	190,915,000
Less: Amount representing interest	[1]		(49,413,000)	(49,413,000)
Senior Notes
Components of short-term borrowings and other debt
Note payable			132,970,000	132,970,000
Repayments on long-term and short-term borrowings
2017			10,340,000	10,340,000
2018			10,143,000	10,143,000
2019			10,143,000	10,143,000
2020			10,143,000	10,143,000
2021			10,143,000	10,143,000
Thereafter			188,715,000	188,715,000
Subtotal			239,627,000	239,627,000
Less: Amount representing interest	[1]		$ (106,657,000)	$ (106,657,000)
Notes Payable to Banks [Member]
Components of short-term borrowings and other debt
Short-term Debt, Percentage Bearing Variable Interest Rate			3.75%	3.75%
Debt Instrument, Face Amount			$ 28,200,000.0	$ 28,200,000.0
Interest Expense, Debt				700,000
Insurance Note [Member]
Components of short-term borrowings and other debt
Interest rate											1.50%
Debt Instrument, Periodic Payment, Principal				200,000
Short-term Debt, Maximum Amount Outstanding During Period				2,100,000
Loans
Repayments on long-term and short-term borrowings
2017			0	0
2018			28,901,000	28,901,000
2019			0	0
2020			0	0
2021			0	0
Thereafter			0	0
Subtotal			28,901,000	28,901,000
Less: Amount representing interest	[1]		(780,000)	(780,000)
Insurance Note and Aircraft [Member]
Repayments on long-term and short-term borrowings
2017			2,032,000	2,032,000
2018			3,410,000	3,410,000
2019			1,363,000	1,363,000
2020			2,512,000	2,512,000
2021			0	0
Thereafter			0	0
Subtotal			9,317,000	9,317,000
Less: Amount representing interest	[1]		(1,070,000)	(1,070,000)
Aircraft and related equipment
Components of short-term borrowings and other debt
Other Notes Payable			8,200,000	8,200,000	14,400,000
Interest Expense, Debt				700,000	800,000
Aircraft and related equipment | Minimum
Components of short-term borrowings and other debt
Interest rate											4.21%
Aircraft and related equipment | Maximum
Components of short-term borrowings and other debt
Interest rate											7.25%
Common Stock Class A
Components of short-term borrowings and other debt
Class of Warrant, Exercise Price of Warrants or Rights | $ / shares												$ 28.72
Convergex Group [Member]
Components of short-term borrowings and other debt
Capital Lease Obligations Incurred			800,000
2022 convertible note [Member] | Convertible Debt
Components of short-term borrowings and other debt
Interest on Convertible Debt, Net of Tax				200,000
Debt Instrument, Convertible, Conversion Price | $ / shares									$ 17.375
Convertible debt, unamortized discount												$ 23,400,000
Amortization of debt discount				200,000
Debt Instrument, Interest Rate, Effective Percentage												7.13%
Debt Issuance Costs, Gross												$ 2,100,000
Debt Instrument, Face Amount									$ 135,000,000.0
2019 convertible note [Member] | Convertible Debt
Components of short-term borrowings and other debt
Interest rate												3.00%
Principal amount of notes being converted												$ 1,000
Interest on Convertible Debt, Net of Tax				4,300,000	4,500,000	4,500,000
Debt Instrument, Convertible, Conversion Price | $ / shares												$ 21.32
Debt Instrument, Repurchase Amount												$ 115,100,000
Convertible debt, unamortized discount												$ 35,700,000
Amortization of debt discount				7,300,000	6,900,000	6,300,000
Debt Instrument, Interest Rate, Effective Percentage												8.89%
Debt Issuance Costs, Gross												$ 900,000
Extinguishment of Debt, Amount				11,600,000
Debt Instrument, Face Amount												$ 149,500,000.0
Cash convertible note hedge transaction					20,500,000
Class of Warrant, Exercise Price of Warrants or Rights | $ / shares												$ 28.72
2027 Notes [Member] | Senior Notes
Components of short-term borrowings and other debt
Interest rate										7.35%
Debt Issuance Costs, Gross										$ 5,100,000
Debt Instrument, Face Amount									$ 18,000,000.0	$ 120,000,000
Interest Expense, Debt, Excluding Amortization				600,000
2021 Notes [Member] | Senior Notes
Components of short-term borrowings and other debt
Extinguishment of Debt, Amount				5,600,000
Debt Instrument, Face Amount			$ 63,250,000.00	$ 63,250,000.00
Debt Instrument, Redemption Price, Percentage				106.188%
Interest Expense, Debt, Excluding Amortization				$ 5,300,000	$ 5,200,000	$ 5,200,000

[1] Amount necessary to reduce net minimum payments to present value calculated at the Company's implicit rate at inception. This amount also includes capitalized debt costs and the unamortized discount on the convertible debt.