Debt Senior Notes (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Jun. 30, 2017
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,400,000
Line of Credit Facility, Expiration Date	Jul. 01, 2022
Line of Credit Facility, Capacity Available for Trade Purchases	$ 50,000
Line of Credit Facility, Accordion Feature	300,000
Senior Notes	100,000
Loss on early extinguishment of debt	$ 0	$ (12,225)
6.5% Senior Notes [Member]
Debt Instrument [Line Items]
Principal		$ 300,000
Stated interest rate, senior notes	6.50%	6.50%
Senior Notes Aggregate Redemption Amount		$ 309,800
Loss on early extinguishment of debt		12,200
Unamortized Premium		9,800
Unamortized Discount		$ 2,400
6% Senior notes [Member]
Debt Instrument [Line Items]
Principal	$ 500,000		$ 500,000	$ 400,000
Senior Notes	$ 495,452		$ 495,308
Stated interest rate, senior notes	6.00%