Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Long-term Debt
Transfers between Levels 1 and 2, description	The Company did not have material transfers between Levels 1 and 2 during the six months ended June 30, 2017 or 2016.
Long-term debt, carrying amount	$ 862,660	$ 759,519
2017 Senior Notes [Member]
Long-term Debt
Long-term debt, fair value	526,300
Long-term debt, carrying amount	492,360
2010 Senior Notes [Member]
Long-term Debt
Long-term debt, fair value		302,600
Long-term debt, carrying amount		298,120
Convertible Notes [Member]
Long-term Debt
Long-term debt, fair value	238,400	228,400
Long-term debt, carrying amount	$ 157,046	$ 152,668