Debt Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 1,200,000	$ 850,000
Credit Agreement	$ 380,000		712,000
Total long-term debt	1,070,615		1,008,752
Six Percent Senior Notes Due Two Thousand Twenty Four [Member] [Member]
Debt Instrument [Line Items]
Principal	400,000
Unamortized discount	(6,712)
6.5% Senior Notes [Member]
Debt Instrument [Line Items]
Principal	300,000		300,000
Unamortized discount	(2,673)		(3,248)
Senior Notes	$ 297,327		$ 296,752
Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 50,000