Long-Term Debt - Outstanding Debt Maturities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Maturities of outstanding debt
2018	$ 28,795
2019	3,291
2020	3,492
2021	703,708
2022	3,174
Thereafter	1,666,392
Total debt	2,408,852	$ 2,386,791
Interest costs
Interest costs incurred	136,100	148,500	$ 154,400
Interest costs incurred capitalized	1,700	2,500	457,000,000
Cash paid for interest	$ 135,381	$ 150,679	$ 160,190