Convertible Debt and Notes Payable (Details)			3 Months Ended		6 Months Ended
		Dec. 05, 2016	Jun. 30, 2019 USD ($) letters_of_credit $ / shares shares	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) letters_of_credit $ / shares shares	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 14, 2017 USD ($) $ / shares	Dec. 08, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 10, 2014 USD ($)
Debt and Capital Lease Obligations [Line Items]
Finance Lease, Right-of-Use Asset, Amortization			$ 299,000	$ 363,000	$ 667,000	$ 715,000
Finance Lease, Interest Expense			56,000	52,000	118,000	97,000
Finance Lease, Interest Payment on Liability					118,000	97,000
Finance Lease, Principal Payments					668,000	1,133,000
Preferred stock dividends			(1,698,000)	(1,698,000)	(3,396,000)	(3,396,000)
Components of short-term borrowings and other debt
Convertible debt	[1]		116,273,000		116,273,000		$ 134,489,000
Note payable			281,544,000		281,544,000		229,740,000
Loans Payable to Bank			28,200,000		28,200,000		28,200,000
Other Notes Payable			1,189,000		1,189,000		0
Capital lease obligations			4,357,000		4,357,000		5,025,000
Debt, Long-term and Short-term, Combined Amount			431,563,000		431,563,000		397,454,000
Purchase of treasury stock, cost					12,015,000
Interest on Convertible Debt, Net of Tax				1,000,000		2,000,000
Amortization of debt discount					2,419,000	2,871,000
Stockholders' Equity Note, Stock Split, Conversion Ratio		0.25
Letter of credit, borrowing capacity			370,000,000		370,000,000
Capital Lease Obligations Incurred					6,000,000
Future minimum lease payments for capital lease obligations
2019			571,000		571,000
2020			1,358,000		1,358,000
2021			1,358,000		1,358,000
2022			1,127,000		1,127,000
2023			374,000		374,000
Thereafter			0		0
Subtotal			4,788,000		4,788,000
Less: Amount representing interest	[2]		(431,000)		(431,000)
Capital lease obligations			$ 4,357,000		$ 4,357,000		5,025,000
Number of letters of credit | letters_of_credit			7		7
Cash collateral pledged			$ 7,097,000	15,513,000	$ 7,097,000	15,513,000	$ 6,318,000
Preferred stock, shares authorized | shares			10,000,000		10,000,000		10,000,000
Preferred stock, par value (in dollars per share) | $ / shares			$ 0.01		$ 0.01		$ 0.01
Finance Lease, Weighted Average Remaining Lease Term			3 years 7 months 28 days		3 years 7 months 28 days
Finance Lease, Weighted Average Discount Rate, Percent			4.93%		4.93%
Letter of Credit, Boston Office, Expires March 2019 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			$ 390,464		$ 390,464
Letter of Credit, NY Office 1, Expires April 2019 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			356,194		356,194
Letter of Credit, NY Office 2, Expires May 2019 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			71,146		71,146
Letter of Credit, NY Office 3, Expires October 2019 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			397,541		397,541
Letter of Credit, NY Office 4, Expires October 2019 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			1,687,045		1,687,045
Letter of Credit, NY Office 5, Expires November 2019 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			1,617,328		1,617,328
Letter of Credit, San Francisco Office, Expires January 2020 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			710,643		710,643
Convertible Debt
Components of short-term borrowings and other debt
Convertible debt, unamortized discount			17,100,000		17,100,000		$ 19,500,000
Repayments on long-term and short-term borrowings
2019			2,025,000		2,025,000
2020			4,050,000		4,050,000
2021			4,050,000		4,050,000
2022			139,050,000		139,050,000
2023			0		0
Thereafter			0		0
Subtotal			149,175,000		149,175,000
Less: Amount representing interest	[2]		(32,902,000)		(32,902,000)
Senior Notes
Components of short-term borrowings and other debt
Note payable			281,544,000		281,544,000
Repayments on long-term and short-term borrowings
2019			10,868,000		10,868,000
2020			21,736,000		21,736,000
2021			21,736,000		21,736,000
2022			21,736,000		21,736,000
2023			21,736,000		21,736,000
Thereafter			407,118,000		407,118,000
Subtotal			504,930,000		504,930,000
Less: Amount representing interest	[2]		(223,386,000)		(223,386,000)
Notes Payable to Banks
Components of short-term borrowings and other debt
Debt Instrument, Face Amount										$ 28,200,000.0
Interest Expense, Debt, Excluding Amortization			$ 400,000.0	400,000.0	$ 900,000.0	800,000.0
Short-term Debt, Percentage Bearing Variable Interest Rate			3.75%		3.75%
Insurance Note
Components of short-term borrowings and other debt
Interest rate			2.51%		2.51%
Short-term Debt, Maximum Amount Outstanding During Period					$ 2,200,000
Debt Instrument, Periodic Payment, Principal					200,000
Loans
Repayments on long-term and short-term borrowings
2019			$ 863,000		863,000
2020			29,034,000		29,034,000
2021			0		0
2022			0		0
2023			0		0
Thereafter			0		0
Subtotal			29,897,000		29,897,000
Less: Amount representing interest	[2]		(1,697,000)		(1,697,000)
Insurance Note and Aircraft [Member]
Repayments on long-term and short-term borrowings
2019			1,214,000		1,214,000
2020			0		0
2021			0		0
2022			0		0
2023			0		0
Thereafter			0		0
Subtotal			1,214,000		1,214,000
Less: Amount representing interest	[2]		$ (25,000)		$ (25,000)
Common Stock Class A
Debt and Capital Lease Obligations [Line Items]
Common stock, shares authorized | shares			62,500,000		62,500,000		62,500,000
Common stock, par value (in dollars per share) | $ / shares			$ 0.01		$ 0.01		$ 0.01
2022 convertible note | Convertible Debt
Components of short-term borrowings and other debt
Convertible debt			$ 135,000,000		$ 135,000,000
Debt Instrument, Face Amount								$ 135,000,000.0
Debt Instrument, Convertible, Conversion Price | $ / shares								$ 17.375
Purchase of treasury stock, cost					19,500,000
Interest on Convertible Debt, Net of Tax			1,000,000		2,000,000
Convertible debt, unamortized discount											$ 23,400,000
Amortization of debt discount			1,100,000	1,000,000	2,100,000	2,000,000
Debt Instrument, Interest Rate, Effective Percentage											7.13%
Debt Issuance Costs, Gross											$ 2,200,000
2019 convertible note | Convertible Debt
Components of short-term borrowings and other debt
Debt Instrument, Face Amount											149,500,000.0
Debt Instrument, Repurchase Amount											$ 115,100,000
Interest on Convertible Debt, Net of Tax				200,000	100,000	700,000
Amortization of debt discount				400,000	300,000	900,000
Debt Instrument, Interest Rate, Effective Percentage											8.89%
Interest rate											3.00%
2024 Notes | Senior Notes
Components of short-term borrowings and other debt
Debt Instrument, Face Amount									$ 53,000,000.0
Debt Issuance Costs, Gross									$ 1,500,000
Interest rate									7.25%
Interest Expense, Debt, Excluding Amortization			600,000		600,000
2033 Notes | Senior Notes
Components of short-term borrowings and other debt
Debt Instrument, Face Amount								$ 10,000,000.0	$ 90,000,000.0
Debt Issuance Costs, Gross									$ 3,600,000
Interest rate									7.75%
Interest Expense, Debt, Excluding Amortization			2,000,000	400,000	3,900,000	400,000
2027 Notes | Senior Notes
Components of short-term borrowings and other debt
Debt Instrument, Face Amount								$ 18,000,000.0	$ 120,000,000
Debt Issuance Costs, Gross									$ 5,000,000
Interest rate									7.35%
Interest Expense, Debt, Excluding Amortization			2,500,000	2,500,000	5,100,000	5,100,000
Collateral Reinsurance Agreement
Future minimum lease payments for capital lease obligations
Cash collateral pledged			1,900,000		1,900,000		$ 1,000,000
Retained Earnings/(Accumulated deficit) | Convertible Preferred Stock
Debt and Capital Lease Obligations [Line Items]
Preferred stock dividends			$ (1,698,000)	$ (1,698,000)	$ (3,396,000)	$ (3,396,000)

[1] The carrying amount of the convertible debt includes an unamortized discount of $17.1 million and $19.5 million as of June 30, 2019 and December 31, 2018, respectively.

[2] 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.