Fair Value - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Total assets and liabilities measured at fair value	$ 20,240,000	$ 20,920,000
Total assets and liabilities measured at fair value priced using non-binding broker quotes	$ 276,800	$ 181,500
Total assets and liabilities measured at fair value priced using non-binding broker quotes (percentage)	1.40%	0.90%
Senior notes	$ 1,733,211	$ 1,732,884
Estimated fair value of senior notes	$ 1,890,000	$ 2,040,000