Investments Available-for-Sale Investments (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Variable Interest Entity, Measure of Activity, Other, Amount	$ 13,400
Available-for-sale Securities, Amortized Cost Basis	22,745	$ 22,745
Available-for-sale Securities, Gross Unrealized Gain		0
Available-for-sale Securities, Gross Unrealized Loss	(2,500)	0
Available-for-sale Securities	20,245	22,745
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	0	0
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	20,245	22,745
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	0	0
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	20,245	22,745
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	2,500	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	20,245	22,745
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 2,500	$ 0