Long-Term Debt and Credit Facility, Textuals (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	3 Months Ended	6 Months Ended			3 Months Ended	6 Months Ended					3 Months Ended		6 Months Ended						3 Months Ended
	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Convertible Notes Days	Jun. 30, 2014 Convertible Notes	Mar. 29, 2010 Convertible Notes	Jun. 30, 2014 Call Options	Jun. 30, 2014 Warrant Transactions	Jun. 30, 2013 Warrant Transactions	Jun. 30, 2014 Senior Notes	Jun. 30, 2013 Senior Notes	Jun. 30, 2014 Senior Notes	Jun. 30, 2013 Senior Notes	May 23, 2012 Senior Notes	Jun. 30, 2014 Revolving Credit Facility	Jun. 30, 2014 Line of Credit	Jun. 30, 2014 Letter of Credit	Jun. 30, 2014 Minimum
Long-term debt
Principal amount of notes	$ 225,000,000	$ 225,000,000		$ 225,000,000			$ 175,000,000								$ 225,000,000
Interest rate							4.50%								8.25%
Interest expense including amortization of deferred financing costs											4,900,000	4,800,000	9,700,000	9,600,000
Payment of quarterly cash dividends	$ 0.35	$ 0.70	$ 0.60																$ 0.24
Conversion rate of common stock shares per $1,000 of principal amount						20.8549
Principal amount in conversion feature					1,000	1,000
Conversion price per share of note					$ 47.95	$ 47.95
Threshold percentage of stock price trigger for conversion					130.00%
Trading days threshold for conversion					20
Consecutive trading days threshold for conversion					30 days
Trading days warrants are exercisable		120 days
Common stock sold related to option counterparties net-share-settled warrants									3.6	3.6
Exercise price per share of Call Options								$ 47.95
Exercise price per share of Warrants									60.89	61.19
Line of Credit Facility
Maximum borrowing capacity																300,000,000
Available borrowing capacity																258,300,000
Outstanding line of credit																	0	(7,400,000)
Remaining available borrowing capacity																$ 250,900,000
Interest on Revolving Credit Facility																4.00%