Debt Senior Notes (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
6.5% Senior Notes [Member]
Debt Instrument [Line Items]
Principal	$ 300,000		$ 300,000
Stated interest rate, senior notes	6.50%
Six Percent Senior Notes Due Two Thousand Twenty Four [Member] [Member]
Debt Instrument [Line Items]
Principal	$ 400,000
Stated interest rate, senior notes	6.00%
Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 50,000