Convertible Debt and Notes Payable (Details)			3 Months Ended		9 Months Ended		12 Months Ended	40 Months Ended
		Dec. 05, 2016	Sep. 30, 2018 USD ($) letters_of_credit	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) letters_of_credit	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jan. 31, 2014 USD ($)	Dec. 14, 2017 USD ($) $ / shares	Dec. 08, 2017 USD ($)	Jan. 31, 2016	Mar. 10, 2014 USD ($) $ / shares
Components of short-term borrowings and other debt
Convertible debt			$ 133,126,000		$ 133,126,000		$ 141,502,000
Note payable			229,581,000		229,581,000		132,970,000
Loans Payable to Bank			28,200,000		28,200,000		28,121,000
Other Notes Payable			7,688,000		7,688,000		8,247,000
Capital lease obligations			5,613,000		5,613,000		4,120,000
Debt, Long-term and Short-term, Combined Amount			404,208,000		404,208,000		314,960,000
Purchase of treasury stock, cost					16,762,000
Amortization of debt discount					4,177,000	$ 5,601,000
Stockholders' Equity Note, Stock Split, Conversion Ratio		0.25
Letter of credit, borrowing capacity			370,000,000		370,000,000
Capital Lease Obligations Incurred								$ 10,100,000
Capital Leases, Income Statement, Interest Expense			100,000	$ 100,000	200,000	100,000
Future minimum lease payments for capital lease obligations
2018			568,000		568,000
2019			1,438,000		1,438,000
2020			1,360,000		1,360,000
2021			1,360,000		1,360,000
2022			1,128,000		1,128,000
Thereafter			374,000		374,000
Subtotal			6,228,000		6,228,000
Less: Amount representing interest	[1]		(615,000)		(615,000)
Capital lease obligations			$ 5,613,000		$ 5,613,000		4,120,000
Number of letters of credit | letters_of_credit			7		7
Cash collateral pledged			$ 16,092,000	16,995,000	$ 16,092,000	16,995,000	17,888,000
Minimum
Components of short-term borrowings and other debt
Capital Lease Obligation, Initial Term					55 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			$ 713,781		$ 713,781
Letter of Credit, Boston Office, Expires March 2019 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			386,022		386,022
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			355,818		355,818
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			70,406		70,406
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			496,563		496,563
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			1,686,297		1,686,297
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			1,615,204		1,615,204
Convertible Debt
Components of short-term borrowings and other debt
Convertible debt, unamortized discount			24,900,000		24,900,000		25,600,000
Repayments on long-term and short-term borrowings
2018			2,025,000		2,025,000
2019			25,223,000		25,223,000
2020			4,050,000		4,050,000
2021			4,050,000		4,050,000
2022			139,050,000		139,050,000
Thereafter			0		0
Subtotal			174,398,000		174,398,000
Less: Amount representing interest	[1]		(41,272,000)		(41,272,000)
Senior Notes
Components of short-term borrowings and other debt
Note payable			229,581,000		229,581,000
Repayments on long-term and short-term borrowings
2018			4,473,000		4,473,000
2019			17,893,000		17,893,000
2020			17,893,000		17,893,000
2021			17,893,000		17,893,000
2022			17,893,000		17,893,000
Thereafter			370,090,000		370,090,000
Subtotal			446,135,000		446,135,000
Less: Amount representing interest	[1]		(216,554,000)		(216,554,000)
Notes Payable to Banks
Components of short-term borrowings and other debt
Debt Instrument, Face Amount			28,200,000.0		28,200,000.0
Interest Expense, Debt, Excluding Amortization			$ 400,000.0	400,000.0	$ 1,200,000.0	400,000.0
Short-term Debt, Percentage Bearing Variable Interest Rate			3.75%		3.75%
Insurance Note
Components of short-term borrowings and other debt
Other Notes Payable			$ 600,000		$ 600,000
Interest rate											2.01%
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
2018			0		0
2019			29,414,000		29,414,000
2020			0		0
2021			0		0
2022			0		0
Thereafter			0		0
Subtotal			29,414,000		29,414,000
Less: Amount representing interest	[1]		(1,214,000)		(1,214,000)
Insurance Note and Aircraft
Repayments on long-term and short-term borrowings
2018			1,106,000		1,106,000
2019			3,410,000		3,410,000
2020			1,363,000		1,363,000
2021			2,512,000		2,512,000
2022			0		0
Thereafter			0		0
Subtotal			8,391,000		8,391,000
Less: Amount representing interest	[1]		(703,000)		(703,000)
Aircraft and related equipment
Components of short-term borrowings and other debt
Other Notes Payable			7,100,000		7,100,000		8,200,000
Interest Expense, Debt			100,000	200,000	300,000	600,000
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%
2022 convertible note | Convertible Debt
Components of short-term borrowings and other debt
Convertible debt			112,900,000		112,900,000
Debt Instrument, Face Amount									$ 135,000,000.0
Debt Instrument, Convertible, Conversion Price | $ / shares									$ 17.375
Interest on Convertible Debt, Net of Tax			1,000,000		3,000,000
Convertible debt, unamortized discount												$ 23,400,000
Amortization of debt discount			1,000,000		3,000,000
Debt Instrument, Interest Rate, Effective Percentage												7.13%
Debt Issuance Costs, Gross												$ 2,200,000
2019 convertible note
Components of short-term borrowings and other debt
Purchase of treasury stock, cost					19,500,000
2019 convertible note | Convertible Debt
Components of short-term borrowings and other debt
Convertible debt			20,200,000		20,200,000
Debt Instrument, Face Amount												$ 149,500,000.0
Debt Instrument, Convertible, Conversion Price | $ / shares												$ 21.32
Debt Instrument, Repurchase Amount			13,500,000		13,500,000		115,100,000					$ 115,100,000
Purchase of treasury stock, cost							300,000
Interest on Convertible Debt, Net of Tax			200,000	1,100,000	900,000	3,400,000
Convertible debt, unamortized discount												$ 35,700,000
Amortization of debt discount			300,000	$ 1,900,000	1,200,000	5,600,000
Debt Instrument, Interest Rate, Effective Percentage												8.89%
Debt Issuance Costs, Gross												$ 900,000
Interest rate												3.00%
Principal amount of notes being converted												$ 1,000
Extinguishment of Debt, Amount					600,000
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										$ 3,800,000
Interest rate										7.75%
Interest Expense, Debt, Excluding Amortization			1,900,000		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		7,600,000
Collateral Reinsurance Agreement
Future minimum lease payments for capital lease obligations
Cash collateral pledged			$ 10,800,000		$ 10,800,000		$ 11,800,000

[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.