Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 30, 2017	Dec. 31, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Shares repurchased, weighted average price per share					$ 8.03
Shares repurchased, value					$ 147,100
Shareholder rights accompanying each outstanding share of the company's common stock (in number of Rights)		1		1
Common stock, beneficial ownership threshold to be considered an Acquiring Person (in hundredths)		5.00%		5.00%
Purchase price (in dollars per share)		$ 45		$ 45
Purchase price (in dollars per one-tenth share)		4.50		4.50
Redemption price (in dollars per Right)		$ 0.001		$ 0.001
Rights
Debt Instrument [Line Items]
Period after person becomes acquiring person				10 days
Common stock purchased per right		0.1		0.1
2% Convertible Senior Notes due 2020
Debt Instrument [Line Items]
Principal amount of notes converted into shares	$ 202,500
Shares issued during period upon conversion of convertible debt (in shares)	29,100,000
Treasury stock reissued during period upon conversion of notes (in shares)	18,700,000
Stock issued during period upon conversion of notes, new issues (in shares)	10,400,000
Repayments of debt (in shares)			18,300,000		18,300,000
Stated interest rate (in hundredths)		2.00%		2.00%
Convertible Junior Subordinated Debentures, at 9% per annum, Due 2063
Debt Instrument [Line Items]
Stated interest rate (in hundredths)		9.00%		9.00%
Reacquisition of convertible junior subordinated debentures-equity component					$ 6,300
Accumulated other comprehensive loss
Debt Instrument [Line Items]
Cumulative effect to reclassify certain tax effects from accumulated other comprehensive loss		$ (10,400)		$ (10,422)
Retained earnings (deficit)
Debt Instrument [Line Items]
Cumulative effect to reclassify certain tax effects from accumulated other comprehensive loss		$ 10,400		$ 10,422