Description of the Business and Significant Accounting Policies (Short-term Investments) (Details)	12 Months Ended
	Dec. 31, 2018 USD ($)
Description of the Business and Significant Accounting Policies
Short-term investments	$ 228,783,000
Certificate of deposits	130,000,000
Commercial paper	59,200,000
US Treasuries	39,600,000
Unrealized losses on short-term investments	$ 32,000