Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended		3 Months Ended	9 Months Ended
	Jul. 23, 2015	Apr. 30, 2017	Oct. 31, 2016	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2017
Class of Warrant or Right [Line Items]
Shares repurchased during period (in shares)			4,800,000	13,500,000
Shares repurchased during period, number of unsettled shares				2,100,000	2,100,000
Shares repurchased, weighted average price per share				$ 8.02
Shares repurchased, value			$ 16.5		$ 108.1
Common stock, beneficial ownership threshold to be considered an Acquiring Person (as a percent)	5.00%
Common shares purchasable per Right (in shares)	0.1
Shareholder rights accompanying each outstanding share of the company's common stock (in number of Rights)	1
Purchase price (in dollars per share)	$ 45
Purchase price, one-tenth share (in dollars per share)	4.50
Redemption price (in dollars per Right)	$ 0.001
Rights
Class of Warrant or Right [Line Items]
Number of rights per outstanding share of common stock (in shares)	1
Period after public announcement that a person has become an acquirer (in days)	10 days
Period after a person announces a tender offer which would make them an acquirer (in days)	10 days
2% Notes
Class of Warrant or Right [Line Items]
Stated interest rate (in hundredths)						2.00%
Principal amount of notes converted into shares		$ 202.5
Convertible Senior Notes
Class of Warrant or Right [Line Items]
Stated interest rate (in hundredths)						5.00%
Convertible Senior Notes | 2% Convertible Senior Notes
Class of Warrant or Right [Line Items]
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 (in shares)		10,400,000
Shares issued for repayment of debt (in shares)				18,300,000
9% Debentures
Class of Warrant or Right [Line Items]
Stated interest rate (in hundredths)						9.00%
Reacquisition of convertible junior subordinated debentures-equity component					$ 6.3