Letters of Credit	12 Months Ended
	Dec. 31, 2017
Financial Instruments Pledged as Collateral [Abstract]
Letters of Credit	Letters of CreditAs of December 31, 2017 and 2016, the Company had a $35.0 million and $25.2 million unsecured standby letter of credit facility, of which $17.5 million and $19.7 million had been drawn upon for issued letters of credit, respectively.