Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 879,112	$ 909,781	$ 856,396	$ 765,439	$ 663,062	$ 610,506	$ 564,245	$ 605,253	$ 3,410,728	$ 2,443,066	$ 1,865,045
Operating income	20,408	4,894	28,983	27,656	21,605	26,716	16,451	29,189	81,941	93,961	(19,649)
Net income (loss)	$ 13,886	$ (5,822)	$ 16,178	$ 15,185	$ 19,005	$ 18,824	$ 7,006	$ 27,786	$ 39,427	$ 72,621	$ (45,835)
Net income (loss) per share
Basic (USD per share)	$ 0.30	$ (0.13)	$ 0.35	$ 0.33	$ 0.41	$ 0.41	$ 0.15	$ 0.60	$ 0.85	$ 1.58	$ (1.08)
Diluted (USD per share)	$ 0.30	$ (0.13)	$ 0.35	$ 0.33	$ 0.41	$ 0.41	$ 0.15	$ 0.58	$ 0.85	$ 1.57	$ (1.08)