The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.3(a))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 325
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=75031664&loc=d3e22529-158502

Name:	us-gaap_RealizedInvestmentGainsLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Employee Benefit Plan - (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
Employee Benefit Plan - (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2014	Dec. 31, 2017
Retirement Benefits [Abstract]
Assets contributed to 401(k) Plan			$ 7,900
Restricted investments	$ 3,392			$ 4,469
401(k) Plan distribution period (in years)	3 years
Compensation expense	$ 1,100	$ 1,200
Investment [Line Items]
Gain (loss) on investment	1,078	(3,122)
Restricted investments
Investment [Line Items]
Gain (loss) on investment	$ 100	$ (100)