Long-Term Debt and Credit Facility, Textuals (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	3 Months Ended							3 Months Ended						3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 Convertible Notes Days	Mar. 29, 2010 Convertible Notes	Mar. 31, 2014 Call Options	Mar. 31, 2014 Warrant Transactions	Mar. 31, 2013 Warrant Transactions	Mar. 31, 2014 Senior Notes	Mar. 31, 2013 Senior Notes	May 23, 2012 Senior Notes	Mar. 31, 2014 Revolving Credit Facility	Mar. 31, 2014 Line of Credit	Mar. 31, 2014 Letter of Credit	Mar. 31, 2014 Minimum
Debt Instrument [Line Items]
Fair Value Assumptions, Expected Dividend Payments														$ 0.24	[1]
Long-term debt
Principal amount of notes				$ 175,000,000						$ 225,000,000
Interest rate				4.50%						8.25%
Interest expense including amortization of deferred financing costs								4,800,000	4,800,000
Payment of quarterly cash dividends	$ 0.35	$ 0.30												$ 0.24
Conversion rate of common stock shares per $1,000 of principal amount			20.8234
Principal amount in conversion feature			1,000
Conversion price per share of note			$ 48.02
Threshold percentage of stock price trigger for conversion			130.00%
Trading days threshold for conversion			20
Consecutive trading days threshold for conversion			30 days
Common stock sold related to option counterparties net-share-settled warrants						3.6	3.6
Exercise price per share of Call Options					$ 48.02
Exercise price per share of Warrants						60.98	61.25
Line of Credit Facility
Maximum borrowing capacity											300,000,000
Available borrowing capacity											263,400,000
Outstanding line of credit												0	7,000,000
Remaining available borrowing capacity											$ 256,400,000
Interest on Revolving Credit Facility											4.00%

[1] The quarterly dividend in the first quarter of 2014 was $0.35 per share, but the model assumes a discrete $0.24 per share quarterly dividend as was paid at the inception of the Call Options. Quarterly dividends in excess of $0.24 per share do not affect the Call Options’ value due to anti-dilution adjustments.