Debt Convertible Debentures (Details) (USD $)	12 Months Ended			1 Months Ended	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2014	Mar. 31, 2015	Jun. 30, 2014	Nov. 15, 2011
Debt Instrument [Line Items]
Aggregate principal amount of debentures outstanding	$ 732,500,000
Payments of Debt Issuance Costs	10,100,000	22,800,000	6,900,000
Convertible Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Aggregate principal outstanding of convertible junior subordinated debentures	732,500,000
Debt instrument periodic payment interest rate	4.75%
Convertible debt condition one for debt conversion	140.00%
Conversion ratio					17.4967
Redemption price, percentage	100.00%
Debt conversion, original debt amount					1,000
Conversion price	$ 74.30
Stock price trigger	$ 80.02
Current conversion amount per principal amount of debentures	57.15
Period of redemption of debentures	at least 20 out of the 30 consecutive trading days immediately prior to the date on which notice of redemption is given
Convertible debt conversion period description	130.00%
Maximum percentage of the Company's net cash proceeds from the sale of its common stock and rights to acquire common stock which can be used for repayment of debt	400.00%
Carrying amount of equity component	215,400,000	215,400,000
Aggregate principal amount of debentures outstanding	732,500,000	732,500,000
Debt instrument, unamortized discount	(350,200,000)	(352,800,000)
Net carrying amount	382,300,000	379,700,000
Interest rate	4.90%	4.90%	4.90%
Interest expense - contractual interest coupon	34,800,000	34,800,000	34,800,000
Amortization of debt discount	2,600,000	2,300,000	2,100,000
Remaining discount amortization period	27 years
Payments of Debt Issuance Costs				10,100,000
Refinancing charges classified as interest expense	1,600,000					1,600,000
7.875% Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate - percentage	7.875%
Face amount of senior notes							250,000,000
Minimum [Member] | Convertible Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Number of days percentage closing common stock price exceeds conversion price condition to convert debentures	20
Maximum [Member] | Convertible Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Number of days percentage closing common stock price exceeds conversion price condition to convert debentures	30
Amount Per Principal Amount [Member] | Convertible Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Payments of Debt Issuance Costs				15
Principal Amount [Member] | Convertible Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Payments of Debt Issuance Costs				$ 1,000