Convertible Debt and Notes Payable (Details)			3 Months Ended		9 Months Ended
		Dec. 05, 2016	Sep. 30, 2019 USD ($) letters_of_credit $ / shares shares	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) letters_of_credit $ / shares shares	Sep. 30, 2018 USD ($)	May 07, 2019 USD ($)	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	$ 409,000	$ 966,000	$ 1,124,000
Finance Lease, Interest Expense			54,000	64,000	172,000	167,000
Finance Lease, Interest Payment on Liability					172,000	167,000
Finance Lease, Principal Payments					951,000	1,630,000
Preferred stock dividends			(1,698,000)	(1,698,000)	(5,094,000)	(5,094,000)
Components of short-term borrowings and other debt
Convertible debt	[1]		117,469,000		117,469,000			$ 134,489,000
Note payable			306,645,000		306,645,000			229,740,000
Loans Payable to Bank			32,180,000		32,180,000			28,200,000
Other Notes Payable			395,000		395,000			0
Capital lease obligations			4,066,000		4,066,000			5,025,000
Debt, Long-term and Short-term, Combined Amount			460,755,000		460,755,000			397,454,000
Purchase of treasury stock, cost					22,420,000
Interest on Convertible Debt, Net of Tax				1,000,000		3,000,000
Amortization of debt discount					3,511,000	4,177,000
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			231,000		231,000
2020			1,356,000		1,356,000
2021			1,356,000		1,356,000
2022			1,125,000		1,125,000
2023			374,000		374,000
Thereafter			0		0
Subtotal			4,442,000		4,442,000
Less: Amount representing interest	[2]		(376,000)		(376,000)
Capital lease obligations			$ 4,066,000		$ 4,066,000			5,025,000
Number of letters of credit | letters_of_credit			7		7
Cash collateral pledged			$ 18,609,000	16,092,000	$ 18,609,000	16,092,000		$ 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 4 months 28 days		3 years 4 months 28 days
Finance Lease, Weighted Average Discount Rate, Percent			4.93%		4.93%
Letter of Credit, Boston Office, Expires March 2020 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			$ 391,943		$ 391,943
Letter of Credit, NY Office 1, Expires April 2020 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			357,543		357,543
Letter of Credit, NY Office 2, Expires October 2019 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			71,416		71,416
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			397,591		397,591
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			1,125,056		1,125,056
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			1,623,455		1,623,455
Letter of Credit, San Francisco Office, Expires October 2025 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			713,335		713,335
Convertible Debt
Components of short-term borrowings and other debt
Convertible debt, unamortized discount			16,000,000		16,000,000			$ 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]		(31,706,000)		(31,706,000)
Senior Notes
Components of short-term borrowings and other debt
Note payable			306,645,000		306,645,000
Repayments on long-term and short-term borrowings
2019			7,301,000		7,301,000
2020			23,548,000		23,548,000
2021			23,548,000		23,548,000
2022			23,548,000		23,548,000
2023			23,548,000		23,548,000
Thereafter			433,025,000		433,025,000
Subtotal			534,518,000		534,518,000
Less: Amount representing interest	[2]		(227,873,000)		(227,873,000)
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			$ 500,000.0	400,000.0	$ 1,400,000.0	1,200,000.0
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			$ 489,000		489,000
2020			33,126,000		33,126,000
2021			0		0
2022			0		0
2023			0		0
Thereafter			0		0
Subtotal			33,615,000		33,615,000
Less: Amount representing interest	[2]		(1,435,000)		(1,435,000)
Insurance Note and Aircraft [Member]
Repayments on long-term and short-term borrowings
2019			405,000		405,000
2020			0		0
2021			0		0
2022			0		0
2023			0		0
Thereafter			0		0
Subtotal			405,000		405,000
Less: Amount representing interest	[2]		$ (10,000)		$ (10,000)
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		3,000,000
Convertible debt, unamortized discount												$ 23,400,000
Amortization of debt discount			1,100,000	1,000,000	3,200,000	3,000,000
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	900,000
Amortization of debt discount				300,000	300,000	1,200,000
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 Instrument, Unamortized Premium			500,000		500,000
Debt Issuance Costs, Gross			600,000		600,000		$ 1,500,000
Interest rate							7.25%
Debt Instrument, Additional amount issued			25,000,000		25,000,000
Interest Expense, Debt, Excluding Amortization			900,000		1,500,000
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			1,900,000	1,900,000	5,800,000	2,300,000
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	7,600,000	7,600,000
Collateral Reinsurance Agreement
Future minimum lease payments for capital lease obligations
Cash collateral pledged			13,900,000		13,900,000			$ 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)	$ (5,094,000)	$ (5,094,000)

[1] The carrying amount of the convertible debt includes an unamortized discount of $16.0 million and $19.5 million as of September 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.