Line of Credit (Details)		1 Months Ended	6 Months Ended
	Dec. 31, 2014 USD ($) item	Sep. 30, 2013 USD ($)	Jun. 30, 2015 USD ($) letter	Jan. 31, 2014 USD ($)
Line of Credit Facility [Line Items]
Line of credit, outstanding amount	$ 28,000,000
Amended And Restated Credit Agreement
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity	$ 150,000,000	$ 150,000,000	$ 150,000,000
Line of credit, percentage of annual fee			0.50%
Loan agreement, interest rate at period end	4.25%		4.25%
Available credit facility	$ 103,100,000		$ 131,100,000
Standby Letters Of Credit
Line of Credit Facility [Line Items]
Letter of credit, outstanding amount	$ 18,900,000		$ 18,900,000
Letters of credit, number	2		2
Silicon Valley Bank
Line of Credit Facility [Line Items]
Commitment fee		$ 430,000
Line of credit facility, one-time arrangement fee percentage		0.30%
Line of credit facility, annual administration fee paid	$ 45,000
Silicon Valley Bank | Standby Letters Of Credit
Line of Credit Facility [Line Items]
Letter of credit, outstanding amount		$ 15,000,000		$ 3,900,000