Long-Term Debt and Credit Facility, Textuals (Details) - USD ($) $ / shares in Units, shares in Millions				1 Months Ended	3 Months Ended			9 Months Ended
	Jul. 01, 2015	Apr. 01, 2015	May. 23, 2012	Oct. 19, 2015	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Mar. 29, 2010
Long-term debt
Repayments of Unsecured Debt								$ 6,100,000	$ 0
Long-term debt					$ 218,900,000			218,900,000		$ 225,000,000
Debt Instrument, Unamortized Discount										2,500,000
Derivative Asset					$ 300,000			$ 300,000		$ 85,700,000
Payment of quarterly cash dividends								$ 1.20	$ 1.05
Warrant Transactions
Long-term debt
Common stock sold related to option counterparties net-share-settled warrants					1.7		3.7	1.7	3.7	3.7
Trading days warrants are exercisable	120 days
Exercise price per share of Warrants					$ 60.32		$ 60.80	$ 60.32	$ 60.80
Senior Notes
Long-term debt
Principal amount of notes			$ 225,000,000
Interest rate			8.25%
Percentage of principal amount issued			100.00%
Interest expense including amortization of deferred financing costs					$ 4,800,000		$ 4,800,000	$ 14,500,000	$ 14,500,000
Repayments of Unsecured Debt					6,100,000
Loss on Repurchase of Debt Instrument					600,000
Long-term debt					218,900,000			218,900,000
Revolving Credit Facility
Line of Credit Facility
Maximum borrowing capacity					300,000,000			300,000,000
Available borrowing capacity					246,500,000			246,500,000
Remaining available borrowing capacity					$ 239,100,000			$ 239,100,000
Interest on Revolving Credit Facility					4.00%			4.00%
Line of Credit
Line of Credit Facility
Outstanding line of credit					$ 0			$ 0
Letter of Credit
Line of Credit Facility
Outstanding line of credit					$ 7,400,000			$ 7,400,000
Minimum
Long-term debt
Payment of quarterly cash dividends					$ 0.24
Convertible Debt
Long-term debt
Principal amount of notes											$ 175,000,000
Interest rate											4.50%
Debt Instrument, Redemption Price, Percentage						154.261%
Consecutive trading days threshold for conversion						50 days
Short-term Debt, Fair Value		$ 273,800,000
Debt Instrument, Final Interest Payment		3,900,000
Principal amount in conversion feature		175,000,000
Debt Instrument, Convertible, If-converted Value in Excess of Principal		94,900,000
Derivative Asset		94,900,000
Net cash outflow in convertible debt settlement		$ 178,900,000
Subsequent Event | Senior Notes
Long-term debt
Repayments of Unsecured Debt				$ 21,100,000
Percentage of principal amount repurchased				107.50%