Long-Term Debt and Credit Facility, Textuals (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	9 Months Ended			3 Months Ended	9 Months Ended					3 Months Ended		9 Months Ended						3 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 Convertible Notes Days	Sep. 30, 2014 Convertible Notes	Mar. 29, 2010 Convertible Notes	Sep. 30, 2014 Call Options	Sep. 30, 2014 Warrant Transactions	Sep. 30, 2013 Warrant Transactions	Sep. 30, 2014 Senior Notes	Sep. 30, 2013 Senior Notes	Sep. 30, 2014 Senior Notes	Sep. 30, 2013 Senior Notes	May 23, 2012 Senior Notes	Sep. 30, 2014 Revolving Credit Facility	Sep. 30, 2014 Line of Credit	Sep. 30, 2014 Letter of Credit	Sep. 30, 2014 Minimum
Long-term debt
Principal amount of notes	$ 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										4,800,000	4,800,000	14,500,000	14,500,000
Payment of quarterly cash dividends	$ 1.05	$ 0.90																$ 0.24
Conversion rate of common stock shares per $1,000 of principal amount					20.8862
Principal amount in conversion feature				1,000	1,000
Conversion price per share of note				$ 47.88	$ 47.88
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								3.7	3.6
Exercise price per share of Call Options							$ 47.88
Exercise price per share of Warrants								60.80	61.14
Line of Credit Facility
Maximum borrowing capacity															300,000,000
Available borrowing capacity															271,000,000
Outstanding line of credit																0	(7,700,000)
Remaining available borrowing capacity															$ 263,300,000
Interest on Revolving Credit Facility															4.00%