Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Level 3 Inputs) (Details) - Other assets and (liabilities) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of the period	$ (2,952)	$ (13,004)
Total realized and unrealized gains
Purchases	289	112
Balance at end of the period	(2,018)	(9,502)
Other income (loss)
Total realized and unrealized gains
Included in other income (loss)	$ 645	$ 3,390