Revenue (Change in Performance Obligation) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amounts included in contract liabilities at the beginning of the period:
Performance obligations satisfied									$ 571
Changes in the period:
Changes in the estimated transaction price allocated to performance obligations satisfied in prior periods									(1,100)
Performance obligations satisfied from new activities in the period - contract revenue									472,713
Total revenues	$ 115,065	$ 113,971	$ 116,508	$ 126,640	$ 118,080	$ 114,846	$ 117,132	$ 126,037	$ 472,184	$ 476,095	$ 355,561