Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income	$ 447,221	$ 602,703	$ 494,090
Other comprehensive income, net of tax:
Change in value of derivatives	81	83	81
Other comprehensive income	81	83	81
Comprehensive income	$ 447,302	$ 602,786	$ 494,171