Commitments and Contingencies (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Rental expense for operating leases	$ 2,900,000	$ 2,300,000	$ 7,200,000	$ 7,300,000
Defined benefit plan employer matching contribution on first five percent of employee compensation			50.00%
Percentage of employees' eligible compensation for employer matching contribution			5.00%
Defined benefit plan employer contribution limit			$ 6,000
Defined benefit plan employer contribution vesting period			5 years
Defined benefit plan employer contribution	400,000	$ 300,000	$ 1,200,000	$ 1,100,000
Letter of Credit
Line of Credit Facility [Line Items]
Proceeds from amounts drawn against letters of credit			0
Letter of Credit | LOC for Office Space
Line of Credit Facility [Line Items]
Letters of credit outstanding	1,800,000		1,800,000
Letter of Credit | LOC for Security Deposit
Line of Credit Facility [Line Items]
Letters of credit outstanding	$ 100,000		$ 100,000