Summary Of Business and Significant Accounting Policies (Narrative) (Details)	12 Months Ended			17 Months Ended
	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)	Jan. 01, 2017 USD ($)
Summary Of Business And Significant Accounting Policies [Line Items]
Impairment of long-lived assets	$ 0	$ 0	$ 3,700,000
Number of reporting units | segment	1
Number of operating segments | segment	1
Goodwill impairment	$ 0	0
Unrealized loss on marketable securities	400,000	354,000	$ 0	$ (1,000,000)
Income tax benefit, marketable securities	200,000
Excess tax benefits	400,000	15,800,000
Increase in total assets	1,785,153,000	1,651,983,000		1,785,153,000
Increase to retained earnings	$ 105,642,000	72,741,000		105,642,000
Minimum
Summary Of Business And Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives	3 years
Finite-lived intangible assets, useful life	1 year
Maximum
Summary Of Business And Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives	5 years
Finite-lived intangible assets, useful life	10 years
Software and software development costs
Summary Of Business And Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives	4 years
ASU 2016-09
Summary Of Business And Significant Accounting Policies [Line Items]
Increase to deferred tax assets		$ 3,700,000
Increase to retained earnings							$ 3,700,000
Subsequent Event | Scenario, Forecast | ASU 2016-02 | Minimum
Summary Of Business And Significant Accounting Policies [Line Items]
Operating and finance lease right of use assets expected to be recognized upon adoption					$ 25,000,000
Operating and finance lease liabilities expected to be recognized upon adoption					35,000,000
Subsequent Event | Scenario, Forecast | ASU 2016-02 | Maximum
Summary Of Business And Significant Accounting Policies [Line Items]
Operating and finance lease right of use assets expected to be recognized upon adoption					40,000,000
Operating and finance lease liabilities expected to be recognized upon adoption					$ 45,000,000
Adjustments | ASU 2014-09
Summary Of Business And Significant Accounting Policies [Line Items]
Increase in total assets	$ (8,776,000)			(8,776,000)		$ 9,300,000
Increase to retained earnings	$ 6,578,000			$ 6,578,000		6,900,000
Increase in deferred tax liabilities						$ 2,400,000