Summarized Quarterly Data (Unaudited) (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	$ 1,155,269	$ 789,651	$ 826,913	$ 816,085	$ 769,036	$ 806,247	$ 796,463	$ 776,224	$ 3,587,918	$ 3,147,970	$ 3,034,380
Operating income	124,360	155,843	181,755	172,134	26,775	143,663	134,989	140,287	634,092	445,714	543,016
Net income (loss)	$ (42,036)	$ 36,125	$ 53,988	$ 45,437	$ (338,060)	$ 789,340	$ 17,079	$ 5,104	$ 93,514	$ 473,463	$ 109,310
Earnings (loss) per common share:
Basic earnings (loss) per common share (in dollars per share)	$ (0.37)	$ 0.39	$ 0.59	$ 0.50	$ (3.72)	$ 8.68	$ 0.19	$ 0.06	$ 0.96	$ 5.21	$ 1.21
Diluted earnings (loss) per common share (in dollars per share)	$ (0.37)	$ 0.38	$ 0.57	$ 0.48	$ (3.72)	$ 8.43	$ 0.18	$ 0.06	$ 0.93	$ 5.07	$ 1.19