Recent Accounting Pronouncements (Summary of Adjustments Made to Consolidated Statements of Income Due to Adoption of New Accounting Standards) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating income	$ 27,831	$ 25,815
ASU No. 2017-07 [Member] | Previously Reported [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating income		23,909
ASU No. 2017-07 [Member] | Effect of Change [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating income		$ 1,906