ºÚÁÏ³Ô¹ÏÍø Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Adoption of ASU 2017-07 (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]	Ìı	Ìı	Ìı	Ìı
Operating income	$ 62,959	$ 49,306	$ 182,945	$ 108,575
Other income	Ìı	18	Ìı	31
As reported in prior year	Ìı	Ìı	Ìı	Ìı
New Accounting Pronouncements or Change in Accounting Principle [Line Items]	Ìı	Ìı	Ìı	Ìı
Operating income	Ìı	39,071	Ìı	85,907
Other income	Ìı	20	Ìı	39
Retrospective Adjustment \| Accounting Standards Update 2017-07	Ìı	Ìı	Ìı	Ìı
New Accounting Pronouncements or Change in Accounting Principle [Line Items]	Ìı	Ìı	Ìı	Ìı
Operating income	Ìı	10,235	Ìı	22,668
Other income	Ìı	$ (2)	Ìı	$ (8)