Summary of Business and Significant Accounting Policies (Schedule of Cash and Cash Equivalents) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents, unrestricted	$ 770,036	$ 779,345
Cash and cash equivalents, restricted	333	332
Total unrestricted and restricted cash and cash equivalents	$ 770,369	$ 779,677	$ 799,579	$ 672,609