Fair Value Measurements Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			$ 38,041
Current period changes in fair value before reclassifications, net of tax	$ 0	$ 36	0	$ 26
Amounts reclassified from accumulated other comprehensive income (loss), net of tax	0	0	0	0
Total other comprehensive income (loss)	0	36	0	26
Balance at end of period	17,530		17,530
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	0	(222)	0	(212)
Balance at end of period	$ 0	$ (186)	$ 0	$ (186)