Basis of Presentation and Significant Accounting Policies - ASU Adoptions (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2018	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification due to TCJA	$ 4,500
Adjustments to retained earnings	419,403			$ 567,510
ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments to retained earnings	1,200
ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments to retained earnings	(300)				$ (344)
ASU 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic benefit cost (income)		$ 31	$ 700
Early Adoption | ASU 2017-12
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments to retained earnings	$ (3)