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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating income	$ 69,417	$ 42,059	$ 97,248	$ 67,874
ASU No. 2017-07 [Member] | Previously Reported [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating income		40,773		64,682
ASU No. 2017-07 [Member] | Effect of Change [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating income		$ 1,286		$ 3,192