Long-Term Debt and Credit Facility, Textuals (Details) (USD $)	3 Months Ended	6 Months Ended			0 Months Ended	6 Months Ended							3 Months Ended		6 Months Ended						3 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jul. 15, 2013 Convertible Notes	Jun. 30, 2013 Convertible Notes	Jun. 28, 2013 Convertible Notes	Mar. 29, 2010 Convertible Notes	Jul. 15, 2013 Call Options	Jun. 30, 2013 Call Options	Jul. 15, 2013 Warrant Transactions	Jun. 30, 2013 Warrant Transactions	Jun. 30, 2013 Senior Notes	Jun. 30, 2012 Senior Notes	Jun. 30, 2013 Senior Notes	Jun. 30, 2012 Senior Notes	May 23, 2012 Senior Notes	Jun. 30, 2013 Revolving Credit Facility	Jun. 30, 2013 Line of Credit	Jun. 30, 2013 Letter of Credit	Jun. 30, 2013 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													4,800,000	2,100,000	9,600,000	2,100,000
Payment of quarterly cash dividends	$ 0.30	$ 0.60	$ 0.50																		$ 0.24
Conversion rate of common stock shares per $1,000 of principal amount					20.7714	20.7522
Principal amount in conversion feature					1,000	1,000
Conversion price per share of note					$ 48.14	$ 48.19
Conversion percentage threshold of stock price over conversion price							130.00%
Number of trading days within 30 consecutive trading days							20 days
Number of consecutive trading days							30 days
Common stock sold related to option counterparties net-share-settled warrants												3,600,000
Exercise price per share of Call Options									$ 48.14	$ 48.19
Exercise price per share of Warrants											61.14	61.19
Line of Credit Facility
Maximum borrowing capacity																		300,000,000
Available borrowing capacity																		266,400,000
Outstanding line of credit																			0	7,000,000
Remaining available borrowing capacity																		$ 259,400,000
Interest on Revolving Credit Facility																		4.00%