Summary of Significant Accounting Policies (Adopted Accounting Pronouncements) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Jan. 01, 2018
New Accounting Pronouncement, Early Adoption [Line Items]
Retained earnings adjustment		$ (419,403)		$ (616,149)
Reclassification of certain income tax effects	[1]	0
Retained Earnings
New Accounting Pronouncement, Early Adoption [Line Items]
Reclassification of certain income tax effects	[1]	(4,518)
Accounting Standards Update 2014-09
New Accounting Pronouncement, Early Adoption [Line Items]
Retained earnings adjustment		(1,200)
Accounting Standards Update 2016-01
New Accounting Pronouncement, Early Adoption [Line Items]
Retained earnings adjustment		300			$ 344
Accounting Standards Update 2017-07
New Accounting Pronouncement, Early Adoption [Line Items]
Pension plan expense		1,700	$ 600
Adjustments for New Accounting Principle, Early Adoption | Accounting Standards Update 2017-12
New Accounting Pronouncement, Early Adoption [Line Items]
Retained earnings adjustment		$ 3

[1] The reclassification from accumulated other comprehensive income (loss) to retained earnings was due to the early adoption of ASU 2018-02, refer to Note 1 for further details on the adoption.