Summary of Significant Accounting Policies (Revenue Adjustment - Balance Sheet) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Deferred commission expense	$ 39,550	$ 39,643	$ 31,443
Liabilities:
Deferred revenue – upfront payments from right-to-use contracts	110,267	108,982	85,596
Equity
Distribution in excess of accumulated earnings	$ (215,749)	(227,166)	(211,980)
Calculated under Revenue Guidance in Effect before Topic 606
Assets
Deferred commission expense			31,443
Liabilities:
Deferred revenue – upfront payments from right-to-use contracts			85,596
Equity
Distribution in excess of accumulated earnings			$ (211,980)
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Assets
Deferred commission expense		8,200
Liabilities:
Deferred revenue – upfront payments from right-to-use contracts		23,386
Equity
Distribution in excess of accumulated earnings		$ (15,186)