Convertible Debt and Notes Payable (Details)			12 Months Ended			40 Months Ended
		Dec. 05, 2016	Dec. 31, 2018 USD ($) letters_of_credit class $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 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]
Preferred stock dividends			$ (6,792,000)	$ (6,792,000)	$ (6,792,000)
Common stock, shares authorized | shares			125,000,000
Components of short-term borrowings and other debt
Convertible debt			$ 134,489,000	141,502,000
Note payable			229,740,000	132,970,000
Loans Payable to Bank			28,200,000	28,121,000
Other Notes Payable			0	8,247,000
Capital lease obligations			5,025,000	4,120,000
Debt, Long-term and Short-term, Combined Amount			397,454,000	314,960,000
Purchase of treasury stock, cost			31,762,000
Interest on Convertible Debt, Net of Tax				200,000
Amortization of debt discount			5,473,000	7,435,000	6,885,000
Stockholders' Equity Note, Stock Split, Conversion Ratio		0.25
Cash convertible note hedge transaction			0	135,000,000	0
Short-term Debt, Maximum Amount Outstanding During Period				2,100,000
Debt Instrument, Periodic Payment, Principal				200,000
Letter of credit, borrowing capacity			370,000,000
Interest Expense, Debt					800,000
Capital Lease Obligations Incurred						$ 10,100,000
Capital Leases, Income Statement, Interest Expense			200,000	100,000	100,000
Future minimum lease payments for capital lease obligations
2019			1,355,000
2020			1,358,000
2021			1,358,000
2022			1,127,000
2023			374,000
Thereafter			0
Subtotal			5,572,000
Less: Amount representing interest	[1]		(547,000)
Capital lease obligations			$ 5,025,000	4,120,000
Number of letters of credit | letters_of_credit			7
Cash collateral pledged			$ 6,318,000	$ 17,888,000	13,342,000
Preferred stock, shares authorized | shares			10,000,000	10,000,000
Preferred stock, par value (in dollars per share) | $ / shares			$ 0.01	$ 0.01
Number of classes | class			2
Minimum
Components of short-term borrowings and other debt
Capital Lease Obligation, Initial Term			55 months
Capital leases, interest rate			3.70%
Maximum
Components of short-term borrowings and other debt
Capital Lease Obligation, Initial Term			60 months
Capital leases, interest rate			5.69%
Letter of Credit, Boston Office, Expires March 2019 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			$ 387,574
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			357,248
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			70,619
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			496,651
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			1,686,539
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			1,605,356
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			716,550
Convertible Debt
Components of short-term borrowings and other debt
Convertible debt, unamortized discount			24,900,000	$ 25,600,000
Repayments on long-term and short-term borrowings
2019			25,092,000
2020			4,050,000
2021			4,050,000
2022			139,050,000
2023			0
Thereafter			0
Subtotal			172,242,000
Less: Amount representing interest	[1]		(37,753,000)
Senior Notes
Components of short-term borrowings and other debt
Note payable			229,740,000
Repayments on long-term and short-term borrowings
2019			17,893,000
2020			17,893,000
2021			17,893,000
2022			17,893,000
2023			17,893,000
Thereafter			352,197,000
Subtotal			441,662,000
Less: Amount representing interest	[1]		(211,922,000)
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			$ 1,600,000.0	$ 700,000.0
Short-term Debt, Percentage Bearing Variable Interest Rate			3.75%
Insurance Note
Components of short-term borrowings and other debt
Interest rate			2.01%	1.50%
Short-term Debt, Maximum Amount Outstanding During Period			$ 2,000,000
Debt Instrument, Periodic Payment, Principal			200,000
Loans
Repayments on long-term and short-term borrowings
2019			29,089,000
2020			0
2021			0
2022			0
2023			0
Thereafter			0
Subtotal			29,089,000
Less: Amount representing interest	[1]		(889,000)
Aircraft and related equipment
Components of short-term borrowings and other debt
Other Notes Payable				$ 8,200,000
Interest Expense, Debt			$ 400,000	$ 700,000
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
Debt and Capital Lease Obligations [Line Items]
Common stock, shares authorized | shares			62,500,000	62,500,000
Common stock, par value (in dollars per share) | $ / shares			$ 0.01	$ 0.01
2022 convertible note | Convertible Debt
Components of short-term borrowings and other debt
Convertible debt			$ 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			4,100,000
Convertible debt, unamortized discount										$ 23,400,000
Amortization of debt discount			4,000,000	$ 200,000
Debt Instrument, Interest Rate, Effective Percentage										7.57%
Debt Issuance Costs, Gross										$ 2,200,000
2019 convertible note | Convertible Debt
Components of short-term borrowings and other debt
Convertible debt			20,800,000
Debt Instrument, Face Amount										$ 149,500,000.0
Debt Instrument, Convertible, Conversion Price | $ / shares										$ 21.32
Debt Instrument, Repurchase Amount			13,500,000	115,100,000						$ 115,100,000
Purchase of treasury stock, cost				300,000
Interest on Convertible Debt, Net of Tax			1,100,000	4,300,000	4,500,000
Convertible debt, unamortized discount										$ 35,700,000
Amortization of debt discount			1,500,000	7,300,000	6,900,000
Debt Instrument, Interest Rate, Effective Percentage										8.89%
Debt Issuance Costs, Gross										$ 3,700,000
Interest rate										3.00%
Principal amount of notes being converted										$ 1,000
Extinguishment of Debt, Amount			600,000	10,500,000
Cash convertible note hedge transaction				20,500,000
Class of Warrant, Exercise Price of Warrants or Rights | $ / shares										$ 28.72
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			4,300,000
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			10,100,000	600,000
2021 Notes [Member] | Senior Notes
Components of short-term borrowings and other debt
Debt Instrument, Face Amount				$ 63,250,000
Debt Instrument, Redemption Price, Percentage				106.188%
Extinguishment of Debt, Amount				$ 5,600,000
Interest Expense, Debt, Excluding Amortization				5,300,000	5,200,000
Collateral Reinsurance Agreement
Future minimum lease payments for capital lease obligations
Cash collateral pledged			1,000,000	11,800,000
Retained Earnings/(Accumulated deficit) | Convertible Preferred Stock
Debt and Capital Lease Obligations [Line Items]
Preferred stock dividends			$ (6,792,000)	$ (6,792,000)	$ (6,792,000)

[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.