Summary of Significant Accounting Policies (Significant Impacts of New Accounting Guidance) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	$ 400,084	$ 373,276	$ 338,943
Net Cash Provided by (Used in) Financing Activities, Continuing Operations	(597,799)	(208,016)	(222,196)
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effective Income Tax Rate Reconciliation, Deduction, Amount	21,500	27,700
Tax Benefits Related To Share Based Payments At Settlement Classified As Operating Cash Flows Instead Of Financing Cash Flows \| Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	21,500	27,700
Net Cash Provided by (Used in) Financing Activities, Continuing Operations	21,500	27,700
Cash Payments To Tax Authorities For Shares Withheld To Meet Tax Withholding Requirements On Restricted Stock Units Are Classified As Financing Cash Flow Instead Of Operating Cash Flow
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	9,400	8,100	4,400
Cash Payments To Tax Authorities For Shares Withheld To Meet Tax Withholding Requirements On Restricted Stock Units Are Classified As Financing Cash Flow Instead Of Operating Cash Flow \| Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Cash Provided by (Used in) Financing Activities, Continuing Operations	$ 9,400	$ 8,100	$ 4,400

Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to deductions. Including, but not limited to, extraterritorial income exclusion deduction, qualified production activity deduction, dividend deduction, deduction for dividend paid to employee stock ownership plan, Medicare prescription drug benefit subsidy deduction, and other deductions.

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Amount of cash inflow (outflow) of financing activities, excluding discontinued operations. Financing activity cash flows include obtaining resources from owners and providing them with a return on, and a return of, their investment; borrowing money and repaying amounts borrowed, or settling the obligation; and obtaining and paying for other resources obtained from creditors on long-term credit.

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Amount of cash inflow (outflow) from operating activities, excluding discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities.

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