Long-Term Debt and Credit Facility, Textuals (Details) (USD $)	3 Months Ended				1 Months Ended	3 Months Ended							3 Months Ended					3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Apr. 15, 2013	Dec. 31, 2012	Apr. 19, 2013 Convertible Notes	Mar. 31, 2013 Convertible Notes	Apr. 15, 2013 Convertible Notes	Mar. 29, 2010 Convertible Notes	Apr. 15, 2013 Call Options	Mar. 31, 2013 Call Options	Apr. 15, 2013 Warrant Transactions	Mar. 31, 2013 Warrant Transactions	Mar. 31, 2013 Senior Notes	May 23, 2012 Senior Notes	Mar. 31, 2013 Revolving Credit Facility	Mar. 31, 2013 Line of Credit	Mar. 31, 2013 Letter of Credit	Mar. 31, 2013 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													4,800,000
Payment of quarterly cash dividends	$ 0.30	$ 0.25																$ 0.24
Conversion rate of common stock shares per $1,000 of principal amount					20.7522	20.7309
Principal amount in conversion feature						1,000	1,000
Conversion price per share of note	$ 48.24		$ 48.19
Common stock sold related to option counterparties net share-settled warrants												3,600,000
Exercise price per share of call options									$ 48.19	$ 48.24
Exercise price per share of Warrants											61.19	61.25
Line of Credit Facility
Maximum borrowing capacity															300,000,000
Available borrowing capacity															286,000,000
Outstanding line of credit																0	6,900,000
Remaining available borrowing capacity															$ 279,100,000
Interest on Revolving Credit Facility															4.00%