Income Taxes (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax benefit resulting from change in tax law	$ 43,484,000	$ 0	$ 0
Unrecognized tax benefits	0	0	0
Income tax interest expense	0	0	0
Income tax penalties expense	0	0	0
Employee's excess tax benefit for stock based compensation	3,397,000	0	$ 0
Operating Loss Carryforwards [Line Items]
Cumulative effect of ASU 2016-09 adoption		7,095,000
Retained Deficit
Operating Loss Carryforwards [Line Items]
Cumulative effect of ASU 2016-09 adoption		$ 7,077,000
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 361,864,000