Fair Value of Financial Instruments - Fair Value of Fixed-Rate Notes (Details) $ in Thousands	Dec. 31, 2017 USD ($)
5.125% Senior Notes due 2019
Fair Value of Fixed - Rate Notes
Fair value of fixed-rate notes	$ 367,997
Fixed interest rate	5.125%
6.875% Senior Notes due 2021
Fair Value of Fixed - Rate Notes
Fair value of fixed-rate notes	$ 374,045
Fixed interest rate	6.875%
7.50% Senior Notes due 2023
Fair Value of Fixed - Rate Notes
Fair value of fixed-rate notes	$ 681,621
Fixed interest rate	7.50%
6.125% Senior Notes due 2025
Fair Value of Fixed - Rate Notes
Fair value of fixed-rate notes	$ 405,804
Fixed interest rate	6.125%