Long-Term Debt (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Schedule of Long-term debt
Fixed rate debt	$ 712,025,000	$ 887,003,000
Total	733,025,000	887,003,000
Revolving Credit Facility [Member]
Schedule of Long-term debt
Revolving credit facility, including short-term unsecured notes	21,000,000
6 3/4% Senior Notes due May 30, 2013 [Member]
Schedule of Long-term debt
Fixed rate debt		175,748,000
8% Senior Notes due November 15, 2016 [Member]
Schedule of Long-term debt
Fixed rate debt	272,025,000	271,255,000
7 3/4% Senior Debenture due June 1, 2027 [Member]
Schedule of Long-term debt
Fixed rate debt	200,000,000	200,000,000
7 1/4% Senior Debenture due September 15, 2027 [Member]
Schedule of Long-term debt
Fixed rate debt	$ 240,000,000	$ 240,000,000